I, recently, replied to a Craiglist ad for a 54 cm Bianchi Giro to replace my vintage 56 cm Nishiki I’m presently riding as I think a 54 cm would be a better fit for me.
My first impression on seeing the bike was that it looked somewhat larger than a 54 cm. A short ride on it confirmed that impression. The seller said that the bike had been was sold to him as a 54 cm, an assertion I had no reason to question. I, then, suggested to measure the length of the seat tube as, from what it’s widely accepted, that is the conventional way to size a bike. In fact we measured 54 cm from the center of the bottom bracket to the top of the tube.

My question to the forum is this: is that measurement in sizing the bike still valid with a bike ( I think it's called "compact") where the top tube slopes towards the seat tube?

By the way, from what I've been able to ascertain, it seems that Bianchi bikes are sized at odd numbers like .......51, 53, 55, 57, and so on. If that's the case I'd guess that the bike in question was probably a 55......