I'm looking for a frame for a road bike so I can customize it with decent parts, and have been doing online research about sizing. I've come up with a broad range of numbers that seem to fall into two groups: a smaller size and a larger size.

My height is 5-9 and my inseam is 31 in.

Here's what I've found (from center of crank to top of seat tube):

About.com -- 56cm
Evanscycles -- 56cm
Bikesizing.org -- 53cm
eBicycles -- 54cm
Sierra Trading -- 53cm
Kleinbikes.com -- 57cm

So is that a 21in bike?

Road and mountain/commuter bikes are sized differently.

This should give you a pretty good idea: https://www.competitivecyclist.com/za...LCULATOR_INTRO

Since you are unsure about bike fit, why not have a pro look you over and get you into the right size, i.e. check things out at a local bike shop. And then buy a bike there. Don't go, have them do a quick sizing, and leave to buy online.

In Trek terms, you'd be on a 54 or 56cm road bike, probably a 20" hybrid, but without being able to evaluate reach to the bars (tt measurement), these are only best guesses.

I am just a bit taller than you so I will give you my 2 cents based on my experience.

Sizing is very confusing. Bikes fit differently. There are sizes and how the frame is measured which don't always match up. Then you add it compact geometry (sloping top tube) and it gets more confusing. Plus if you dont' really know what you like then all this becomes to theoretical.

The thing to do is go to a couple of shops and ride some bikes and figure out what size you want. You might still end up with 2 sizes. I would say you should go for a 56 (22 inches) and maybe a 54 if its a compact geometery but only you can decide.

Good luck sizing is really tough, but if you find the right fit its really great. You might not get it right the first time so don't overthink it too much. There is always another bike. Also you might consider buying a complete bike if this is your first road bike. Its tough to build up a bike and then it dosn't really fit. Its happened to me.

Mountain or road? How do you size hybrid bikes?

I have trouble knowing if the local pro knows what he's doing or just wants to move a bike on the floor. I've been buying my accessories at three different bike shops in Salem, just to try to get an idea of how competent the sales people seem and how willing they seem to help. But it's hard to judge another person's competence in a field concerning which I know almost nothing.

Test ride bikes of various sizes, making sure to ride both on and off the saddle, and then choose the size that works best for you. It is my opinion that both a 54cm or 56cm will work for you. With the 54 you might need a slightly longer stem, depending on your upper body.

Sizes tend to be general predictors. You will get a ball park general size, but then you have to go in and actually see what fits best. May take trying a few bikes too.

As a general example, I'm around 5' 11". In my case, most bike shop pros recommend a 56" road bike size, but I'm far more comfortable with a 54". I actually put a shorter stem on my 56" Allez to get somewhat comfortable, and it is still a compromise. Bike sizing is one of the things in which I highly recommend new cyclist to take their time and try several sizes in several models.

As others have said, sizing is more complicated than "I'm this tall, so I need this size". Some people prefer a larger frame, others prefer the smaller frame and longer stem/seat post, etc...

Conventional wisdom is that it is easier to make a smaller frame 'feel' bigger, than it is to make a larger frame work for a smaller person, so if you're torn between the 54 and 56, and can't ride them both enough to be 'sure' then choose the 54.

I am essentially your reported size (5'9, 31" inseam), but not everyone is comfortable the same way. In my experience, 58cm will be too big (I had a 58cm road bike that I could ride, but it never felt comfortable -- but you could be different). Even the choice between 56 and 54 (or near equivalents) is very personal. Geometry of the bike will matter a lot. Do you like being more 'stretched' or do you want to feel more relaxed? I had a 54cm Dew Plus, and when it walked away, I replaced it with a 56cm Dew Drop. I'm completely comfortable on my 56, but I can see where a slight change in geometry would make another 56 feel a little too big. OTOH, some 54s will feel too snug for me.

Good advice above..... Make sure you go for a long test ride and don't be afraid to let the bike shop fit you and make adjustments as needed. They can do things to adjust the fit even if the frame a little too big/small.

I was in the same spot as you, even the same height/inseam. I decided to buy a higher quality "classic" road bike 56cm ($200 on craigslist), so I could learn what I wanted in a road bike before I bought a new one.

I learned a lot with the old bike. In the meantime, I read Rivendell's philosophy on riding a bike and it made great sense. The gist is most people are on race bikes that are too small for them when what they really want is to be comfortable. Here's the site:

https://www.rivbike.com/article/bike_...izing_position

I couldn't afford a Rivendell, even if I wanted one. I found the Salsa casseroll and vaya had the same philosophy. And I am extemely happy with my 56 vaya.

Bike sizing depends on personal preference and the type of riding you do. I have found that I generally prefer frames a little larger than the general recommendations. I think that the smaller recommendations on road bikes are going for a race/speed fit and the larger sizes are for a more relaxed ride.

With compact geometry, the seat tube length really does not matter the way a flat top tube did back in the day. The seat tube angle, head tube length, front center, and top tube length are going to be the primary measurement you need to consider when you are comparing different sizes and different brand.

Thanks all for responses.
The above link was really helpful, as it includes reach measurement and then gives three riding style sizes.
I'm wanting this bike as a tourist visiting different places around Montreal (which has many bike lanes), but also for exercise (read: need to lose 10lbs and supplement my current workout). I figure while I'm looking for work I might as well do something interesting like start a blog about Montreal's parks, ride to all of them (there are too many), and have some fun.
Anyhow, came up with 56 seat tube, 54 top tube.
Going to look for used bike with those dimensions.

Good luck with that... Most bikes have tt at least equal to st, most are longer. Probably look for bikes with a 54 st measurement, they'll be most likely to have the tt measurement you need.

Ride as many bikes in your size range (54-56cm or 17-19") +/- 2 in either direction. You will be able to tell after a few test rides what you like.

However, there are exceptions. I'm 5'9 and have a 23" rigid MTB that worked well for me.

Agreed.

Also, the seat post allows adjustment, so actual seat tube length matters little, IMHO. Personally, I don't think you will be able to comfortably stand over most 56cm bikes, without a sloping top tube. As others have said, sizing is very personal, but I would think you to be almost a perfect fit for a 54cm road bike, perhaps a size smaller if it is a cyclocross bike with a tall bottom bracket.