Like Twindaddy said, it really depends.
I'm 6' 0" and have a 32" inseam. I have an 18" frame and it fits perfect. I'm pretty confident the 20" would be too big...for me. I have tried out 17", 18" 19" and 20" frames on friends' bikes as well as in bike stores.
- All the 17" bikes feel OK but maybe a little too small. I feel like the seat post has to be set too high to get proper leg extension. This makes me feel like I am too top heavy and always at risk of going over the bars.:eekster:
- I kind of feel like I am on my tiptoes a bit too much on the 19"
- The 20" feels too big. I can't even get both feet down without dropping the seat too low.
I could live with a 17" or a 19" frame on a bike (with 26" wheels) but the 18" seems to be my sweetspot. I'm sure there can also be some variance between different makes and models but I found the 17", 18", 19", and 20" fittings to be surprisingly consistent regardless of brand/model. It is probably worth it to try a few bikes out at your LBS if you haven't already done so. See what feels best regarding size.
The one difference I found when testing out different sizes, brands, and models was a Cannondale F5. I believe their medium frame is only 17" but it seemed to fit pretty much the same as my Asta 18" frame.
The bottom line for me is...
- The 18" is the best fit for my body.
- The 19" is a little too large for me but could work and would be my second choice if there were no 18" option.
- The 17" could also work but not as well as the 18" or even the 19".
- The 20" could work in a pinch - if it was a buddy's bike that he was letting me borrow because I had no other bike to ride and I was really jonesing for a ride - but I would blame any crashes or other mishaps that happened on that ride on the poor fit of the bike.