Is it better to have a bike fit too small or too big? I ask because I have been looking for a bike for my gf. I took her to my LBS, and had her fitted on a trek fx, and we came to the conclusion a 17.5 is best for her. So we went out looked on craigslist and found a trek 7.2 with deore xt shifters and rear derailluer, for only $150. At the time it seemed to fit well, so we bought it. After riding it though we realized it was too big for her, and when I measured it discovered it was a 20 and not a 17.5. I recently came across a specialized Vita Sport frame, that pretty much just needs tires for only $125, but is 15 inch frame. Will she feel better, and get a better fit out of this bike? It's down to these two bike because we can not find anything of the same quality in this price range. My plan would be to swap the tires and components of the two bikes then resell the Trek or maybe keep it around and try building it for myself.

Let me just add she is 5'6 with long legs and a shorter torso.

There is probably more you can do to make a too-small frame to fit than a too-large frame.

+1

One thing I discovered with Craigslist is searching for the FX series is funky. Drop the "7" when searching (i.e., Trek .2FX) and also try the search term "Trek FX". I see a lot of 17.5" Trek FX series up there but I need a 20". If your 7.2FX is not the WSD I'd be interested in taking it off your hands. Here's one in the IE:

https://inlandempire.craigslist.org/bik/2438016962.html

BTW, I'd get a 17.5" and save the headaches of buying adjustable stems and/or riser bars to attempt to make a too small bike fit. Long legs + small frame = uncomfortable riding position.

Yea I usually search "trek fx" instead of a particular model. The reason why I'm really considering buying a 15" model is because it fits perfectly into our budget. I don't think I will get much more than $175 for my current fx, since it kind of scratched up. Every 17.5 I see is at least $300 and an older model than her 2008.

I think a smaller size instead of a bigger one will be better for her because she has trouble getting on and off, and her shoulders ache when riding. Is her shoulder pain due to a too long of a top tube?

Bump..... I can really use an answer about my Gf's shoulder pain.

Yes, the shoulder pain is probably from having to reach too far and a 20" is not recommended for her height. I had the same thing on my road bike before I put a shorter stem on it. For a 5'6" individual with long legs a 17.5" frame is just about perfect. I would not recommend getting the 15" frame since it could result in having an uncomfortable amount of saddle to bar drop.

thank you for the advice. maybe I'LL wait until the perfect opportunity
arrives to upgrade her bike.