I tend to get a 56cm bike for road bikes. But I have varied from that a bit when the deal is right. I play around with stem length to compensate somewhat. My smallest road bike is a 54 cm (too small), while my largest is 58cm. That reminds me I need to swap stems on that 54cm bike.

One challenge with a lot of vintage bikes is that there is quite a spread between sizes. I see a lot of 21 inch and 23 inch frames, but don't see many 22 inch frames. This means you go from a 21 inch frame (53 to 54cm) to a 23 inch frame (58 1/2 cm). Pretty big jump.

Then you have the variation between manufacturers. My 1987 Miyata 215ST (23 inch) fits like a 56 cm bike, while the 23 inch Cannondale T600 I had fit more like a 60 cm bike (I sold it).

I like vintage bikes, so I end up getting a compromise, rather than the "perfect" or ideal size.

Modern bikes tend to come in even cm sizes so you can really dial in the size. So my modern Giant is a 56.