I'm building a touring rig for my son to try out for a four day camping trip. He doesn't want to put much money into a touring bike until he knows if he likes it. I have a 54 cm and a 58 cm touring frames. So, I thought I'd use one of those for his trial. Problem is he's a dead ringer for fitting a 56n cm frame. So, which would be better going small or going big? I'm leaning toward using the 58 cm. Oh, I have plenty of stems, bars and even straight and set back seat posts to get him close to where he should be.

I think it will depend on your son's build which frame would fit him best, primarily the proportion of legs to torso length. What about stand over height?

I'd tend to go the other way, unless using the 54 cm would put the seat too far above the bars. I'm basing this opinion on my on experience of being in between a 56 and a 58 cm frame size. My solution was to replace the fork on a smaller frame with a fork with a longer steerer tube. I have long legs and a short torso. The ability to raise the bars and adjust the saddle/ bar height relationship allowed a nice fit.

I would go 58 with a shorter stem, as long as he can stand over it with one leg on a pedal at a stop. He'll appreciate not being hunched over on a four day trip.

Edit: I should say that all my personal bikes are setup this way due to how I'm built, so its my preference. I have extremely long legs and a very short torso for my height. I'm just over 6' and my mountain bike wound up being a 22"/XL Surly with a 90mm stem, and I still have the seatpost out most of the way.

As you can already see, you going to get different answers, none of us can give a good answer without knowing/seeing your son, his riding style etc.
I mayseld, would normally be fit to a 54, but I have a 'Medium/long frame, closer to a 56, I like being stretched out.
Your son, if he hasn't done a lot of riding may feel better being more upright & like a smaller frame, again, it tuff to make the call on the info you have given, it's like flipping a coin.

Good point. He's 6 ft. on the nose and has an inseam of 33 inches. So, I suspect his legs aren't all the long for his height. He rides about 1000 miles a year, according to the computer on is city bike.

IF his legs are shorter & he still has standover height, he *MAY* like the bigger bike because of the longer toptube (IMHO the toptube is the way a bike should be chosen)
A seat post is (relatively speaking) infinitely adjustable, stems not so much & different (especially) extreme length and/or angled stems can affect the handling.

You know, the more I think about this, the more it goes against what I think should happen. I think I'll spend a day or two seeing if I can swap one or both frames for a 56 and make sure it fits him properly.

Personally, at 6', I'd definitely at least try the 58cm first(assuming you don't find yourself a suitable 56). I'm 5'7" with a slightly shorter inseam than him and ride a 54-55cm. I think he'd either be cramped or have his bars well below the seat on a 54cm. ...Unless that's the position he's comfortable in over long distances.

Personally, I think you should focus more on top-tube length than seat-tube height. Measure from seat-post to hand position on his current bike(assuming it's a good touring height for him). Then try to match that on the other frames. Height can be adjusted fairly easily once you find the frame you want to use.

More Info? Level top tube ? If it slopes the seat tube lengths become less meaningful..

then you start comparing Virtual top Tube lengths.

which of course is ,reach adjusted, with stem lengths ..

If he's really 6ft, then don't go 54, assuming the frame geometry proportions the top tube similar to the seat tube and the frame sizing number, the 58 is likely to be closer than the 54 and you can always go with a short stem.

Contrary to what I posted above, I have to agree with robow due to your son's height.

NOS88 I'm just over 6' and inseam just under 33". My road bike is a Specialized 58.5 and my Surly disc trucker is a 58. Yes the 54 is too small and I test road several 56 frames and felt they were too small for me, one was a used Cannondale Touring 1 (recently tested it). The other 56 I had is a Fuji and I gave it to my son because it was too small. I have a ton of miles on both my 58 cm bikes. Note that I recently bought a used BMC road bike that is listed as 60 but it is more like the 58 Specialized....very similar in size.

I was between sizes when I bought my Aurora. I went for the larger frame and was glad I did.

Im a bit over 5'10 and ride a 54, which suits my torso and all that very well, but dont forget, you can find one 54 or 58 with a given toptube length that will be shorter or longer than another given 54 or 58 frame depending on the brand.
Good luck finding one that he is very comfortable on.