56 to 59cm ST. But how is knowing my nominal frame size going to help you know yours?

The STs are different lengths, but I sure don't recall what they are. The important point is that I've made them all comfortable by selecting a suitable stem length. Then with adjustments to saddle position and handlebar height I've made them all nice to ride.

They feel different. The two that feel the longest are the tandem and the UO8. (I'm shorter than I was when I bought the UO8 43 years ago.) The shortest feeling is the Masi, but then that's the 56cm. I've never figured out how much of the difference is my position and shape on the bike vs. the frame geometry vs. the gearing, etc. But it doesn't matter. When I hop on a bike I haven't ridden in a while it may feel odd at first but within a mile it feels exactly right. The nominal 58cm Bianchi is the only one which feel exactly right immediately every time I get on it.

But I still don't see how this will help you.