HTupolev

I'm 5'11", and on most bikes I fit best on something that's a "size 56" or thereabouts.

The fit on my old Fuji America looks perfectly reasonable. I've got about a fist worth of seatpost showing, I'm using the original stem set about halfway between slammed and fully-extended, and the handlebar ends are pointing roughly at the seatstay brake bridge.
The fit is surprisingly versatile. I find it more comfortable for long days than any other bike in my stable, but it still offers practical aggressive racing postures. Occasionally I ride considerable distances to get to a hard spirited group ride, and sometimes end up at 100+ miles for the day; it's impossible to feel anywhere near "comfortable" after such an ordeal, but the Fuji is probably my favorite tool for that sort of job.

The seat tube on that old Fuji happens to measure around 59cm center-to-center.

There have been a couple times when I've told bike shop employees what the seat tube on the old Fuji measures. On one occasion, this tiny snippet of knowledge caused the guy to suddenly conclude that the bike was too big for me.

Did they make that judgement based on your height and the number "56", or did they make it based on how the bike was actually set up and how you looked on it?