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......

Compact frames with sloping top tubes often have 2 measurements. First is the nominal size, like you measured. There may also be a virtual size which is the equivalent size if the top tube were horizontal. Always ask for lots of different measurements when buying a frame without test riding it first. For most people, the top tube length is more important than the seat tube length.

Also, Italian bikes are often measured to the center of the top tube instead of to the top of the top tube. That can cause a 1cm difference in nominal size.

You're not alone in that thought apparently.

+1, the "virtual" top tube measurement is the one that matters most.

Unfortunately, there is no industry standard for a stated size in bicycles, and compact geometry frames have made it even more complicated. The only way to truly know what you're getting is to find the stated size of the bike and look up the manufacturer's geometry chart for that model.

For a bike that you can test ride, the size is not so important right? If it feels too big, it doesn't matter whether its a 54 or a 56?