I also have a 21" Trek 520! It's a 1992, great bike. It's the smallest frame (as measured by the seat tube) I've ever had. But it's got a long top tube, long stem, and the handlebars extend forward pretty far, so it fits my long torso pretty well. I think it's still a tad short in the seat tube, but with the high BB and big 45mm tires I use, I was afraid the next size up (23") would have been too big. I figured it would be better a bit small than too big. It puts the seat quite a bit higher than the bars, but fortunately I like it that way. The bike is getting to be a classic and I want it to last me a long time. It's held up well through 18 years of daily beatings on the rough streets where I live, but last October I broke the steering tube. I'm thinking of retiring it from the daily grind of commuting and saving it for touring and excursions, and replacing it with an old MTB.