Last year, I participated in the IRO group buy. Assuming that because I had ridden a Trek 2300 for a number of years, sized '54', I would go with the IRO '53' as they both measure 54cm top tube. I am a hair under 5' 10" with a 32 and change cycling inseam. Well, let it now be said that there is way more to fit than just top tube measurement.
I got the 53 IRO and had it built up and fitted to replicate my previous Miyata conversion (which was a close fit but in hindsight, also too small). This picture shows how it looked at first. Notice the giant saddle to bar drop and amount of seatpost.
I rode it for a while like this, tried the stem flipped up, a stack of spacers, but still never got comfortable.
I had an extra road bar around, and thought I'd set it up with road bars and levers to see if I could get the bars a bit higher. (those Nitto bullhorns do have a pretty deep drop.)
Better but still too much saddle to bar drop and too much overall discomfort. So I decided to sell the frame and start over. I called Tony (should have done this on the group buy), and gave him my height and measurements and explained my problem. Tony suggested with confidence, that I ought to be on the 59cm size!!! So I bought a new Mark V, sold the group buy frame and rebuilt.
Now everything is so much better. The top tube is 57cm on the 59 size, but even with that I run a 110 stem. Saddle to bar drop is now only 4 or 5 cm and I'm still using about 3cm of spacers. Perfect for the kind of riding this bike sees. Notice too how much 'lower' the saddle seems on the frame (I have a collar for a blinkie light that makes it look even shorter). Need to get a new seat post that has no setback and it'll be finally done. Lots of headaches for sure.
I think in hindsight, I drank the Kool-Aid about running a slightly smaller bike than ideal. But, IROs just run much smaller than their stated size. A lot of it has to do with the head tube length (or low top tube).
Bottom line is there is way more to bike fit than "get the top tube right, and you'll be good to go".