No offense, but it also sounds like it's not just your stem thats too big, but your whole bike.

5'7" on a 55cm?

FWIW I'm 5'5" with similar proportions

Has no one stated the obvious - or did I just miss it?

You need to fit the bike on your saddle setback - you need to know that number. Reach is totally unimportant until after the setback is established. In fact, everyone should have their saddle height and setback memorized. Set up the bikes with your saddle height and setback for testing. Ok now, what size stem does each bike need to obtain your preferred reach? The bar/saddle drop also comes into play depending on the frame and how many spacers you need/want.

And be aware, in some cases the geometry of a bike can literally make the fit impossible without the use of 30mm+ setback posts (and there are very limited choice there).

While all of this is true, it is not really pertinent to the OPs question

The assumption is that either the 56 or 58 will fit given a reasonable stem length (90-120mm) to maintain the same reach.

Yeah, plan was that i would grow into it. Physician says I will be around 5'10-11''

Trunk and legs proportioned about normal for 6'1", however, wingspan on par with 6'7"

58 or 61 cm?

I went with 58 cm frame and 130mm stem; so far, so good. Never did get to ride both frame sizes on the same day...

I am on a 54cm frame with flipped 90mm stem. just under 5"9 longer legs, shorter torso.
From my experience, going from a 110 mm stem to a 90 made a good difference in my comfort on the bike. To me, every little microadjustment can make a difference in riding comfort. We're talking about two centimeters here. That's big, especially when you consider the weight balance of your butt and hands, ie which is supporting more weight. Perhaps it is nonsense to some, but stem changes of one or more centimeters make a big difference. Moving from a 110mm stem to a 90mm stem would put the handlebars about 3/4 of an inch closer and consquently tilt you up slightly.

I'm the same height with the same (cycling) inseam. I use a 90mm, -8 deg. stem. I use a 0 setback seatpost with the tip of the saddle at about 3 cm behind the BB center. I've got long(ish) arms too, so the slightly long TT doesn't bother me. It's the most comfortable I've ever been on a road bike.

You're lucky to be young & still growing- I stopped about 18 years ago.

I would like you explain how you think I (or the OP) can size a frame based on stem length please. I ride a 58 with a 90mm stem, a 100 stem is VERY uncomfortable (just feels far too stretched on the hoods or the drops).

My seat post is nearly on the limit of extension and with the 56 frame the seat simply could not go high enough.

What size frame should I ride to get a longer stem and a good fit? Im 6'3", most of it in my legs

Oh and I have had a pro fit by very well respected fitter

i'm 5' 8" (measured barefoot) and ride a 51 with a 120mm stem FWIW
i could have gone up a size and it would made little difference

Did your well respected fitter suggest that you should get a tall head-tube/short top-tube style bike? And if not, why not?

No he didnt, I took my existing bike along which has a 205mm head tube + 20mm headset cone up top. For a 58 frame that IS a tall head tube and im still using a 90mm stem. So you see, you cant size a bike based on stem length, you get the seat right and base it on that which is what the OP has found in this case. The most a different size frame will do is raise or lower the bars and at most mean you need to change the stem 1 size, but im already on the smallest possible frame with a long head tube. No frame would ever get me using a 120mm stem (just to satisfy your misguided rule of thumb) for example. Why do they make diffent stems if everyone needs the same length?!

EDIT: Actually come to think of it his first reaction when he saw me standing next to the bike was that it might be too small for me but then he agreed that he would have put me on the same due to the reach and agreed it was the best size frame for me. But oh no you know better because 90mm stems are for people who's frame is too big

Update on my frame size selection:

I picked up the rental bikes tonight and set them up. I spent a lot of time getting the seat position exactly the same as my current bike and then started playing with stems and doing tests on my rollers. I'm still debating, but I'm leaning toward the 56. My only concern with the 56 is that the saddle to bar drop is pretty extreme (more than my current 58cm Tarmac), so I'll need to use significant spacers and may need to run with the stem flipped up. The 58 has the opposite problem (with 35 cm more head tube length), so I have it set with no spacers at this point to get a similar drop to the 56. The reach on the 56 feels great with a 110 stem, while the 58 with a 100cm stem is just a little long (same issue I have with my 58 tarmac).

I plan to do a long ride tomorrow on both bikes, with at least 30-40miles on the 56. If I am comfortable with the saddle to bar drop on the 56 after riding 50+ miles, I'm going to go that route.

Not quite. I have a 90 on a 58cm frame, and the frame is to small for me. And a 100mm stem is too great of a reach

Sounds like you have exacly the same long legs and short body I do. Having been in this exact same place I would go with the 58. A large seat bar drop will get uncomfortable on long rides and a flipped up stem with extra spacers will grate on your nerves with the way it looks. I'll bet whatever you try, you would be happier on the 58 with the stem off your tarmac.

Also, having the seat on the limit of extension isnt ideal. Go with a 90mm stem, there is absolutely nothing wrong with it - it fits you as proven by your tarmac.

After riding about 65 miles today between the 2 bikes, I ended up going with the 58. In order to get the bar drop where I wanted it for long distance comfort, the 56 needed 2cm of spacers and the stem flipped up. I could live with the look of it, but the angle of the stem keeps me from being able to see my computer screen when climbing out of the saddle. I tried the stem flipped down for about 15 miles and it was just too low for long distance comfort.

I was able to set the 58 up almost identically to the 56 and I think it will work well, but will still have a 90mm stem just like my Tarmac. The reach was the only thing we couldn't get an exact match on. I was suprised that the reach on the 58cm with a 90mm stem was still a bit longer (about 3mm) than the reach on the 56cm with a 110mm stem. One thing I really preferred on the 56 was the narrower bars, so I'll be swapping the standard 44cm bars on the 58 for some 42cm bars.

My LBS still had to order the 58, so I should have it late next week.

kinda weird, I had the EXACT same situation. invetween 56 and 58. I do 58 with a 90mm stem bc I like the larger frame. less saddle-handlebar drop. my bianchi is a 57. I just don't like having the saddle jacked up higher than it has to be.

Ooh look the smaller frame was a worse fit and the OP was unable to get completely happy without using a short stem. The LBS sounds like they went out of their way to help the OP and get him happy. Isn't it lucky that everyone ignored you.

I just cant understand why you think everyone needs the same length stem or else there must be a problem. I wish I could understand because a number of people share the same view, but it just doesnt make any sense. At all.

By the way to the OP, enjoy your new bike, I'm glad you got it sorted and hope you have many happy miles on it!

My LBS was really patient with me while I was doing a 'Brett Favre' on frame sizes.

The day after I originally test rode a 56 and special ordered it it, I told them I wasn't sure about the 56 size and wanted to do a long test ride. They said "no problem, let's make sure we get it right". They set me up with a 56 and 58 from their rental fleet. They also kept the order in for the 56 rather than cancelling it (the 56 came in yesterday, I saw it when I was picking up the rentals, it was shiney).

I could probably work a deal on the same bike elsewhere for a little less $, but I feel I am getting great value given the level of service.

For anyone looking for a bike shop in Austin, I highly recommend Bicycle Sport Shop. They are a big shop with 2 locations, but you can definitely get personal service.

Congratulations on your new bike!

I am between sizes as well, and it only took me 3 bikes to find the right one! You are ahead of the game.

There is nothing wrong with shorter stems, I ride a medium size Look 566 (I'm 5 feet 7 inches) and upon fitting got a 80mm stem and the ride is really good (Could have opted for a 90mm stem but wanterd a more upright posture anyway)

My commuter bike is the same size as my Lemond Zurich road bike and I'm 5'7" with the same inseam as the other poster. The top tube length is more critical than the seat tube height and on Lemond, its quite long and one may need a taller stem to bring one closer to the handlebars.

grwoolf,

I have the exact same height and inseam measurements as you, and after a long time fiddling with my fit one small piece at a time, I am currently riding a frame with a 550mm effective top tube and ~175mm effective head tube with no spacers. When I want to be in a low and long position for shorter fast rides, I use a 0-degree 130mm stem. For longer rides where I want to be more comfortable I use a 6-degree 110mm stem (actually, this is what I use most of the time). Unless you have long arms, I would say 58cm is on the large side of things for you especially given the taller head tubes found on the Tarmacs (the ones without the pro geometry). But this may not be a bad thing if you like/need a minimal amount of saddle to handlebar drop and don't mind using a shorter stem.