Standard frame sizing is anything but standard. Typically most diamond frame bikes are measure C/C but some are center to top. A 56cm C/C is larger than a 56cm C/T. I prefer to measure the top tube length which is always C/C and is often a better measurement for sizing. Many old bikes are "square" in that the seat tube C/C and the top tube C/C are the same or very similar.

Lots of people like to ride bikes that I would say are too big for them but they would likely think mine are too small for me. I am approximately 5"11" (just under and shrinking) and prefer a 56cm C/C frame with the same top tube and a 100mm to 110mm stem.

Some vintage bikes like the E3s that I have recently become infatuated with yet again are only available in 21 inch and 23 inch and the 23 inch feels like a boat, like wearing a shoe two sizes too big. But, in reading through the threads, many fellows ride giant frames (once actually a style) with the saddle slammed down and the bars jacked up over the saddle with extra long stems. And they like it that way. My point is that fit is personal in that your use for the bike, your preferences may vary from mine or even from one style of bike to another.

That said, you may have a bike a little large and in the future, unless you are collecting, avoid 27 inch wheels, look for 700c frames, usually better quality, more tire sizes, it is the standard for street bikes and now mtbs (29ers). I always think of 27 inch as low end department store crap. I know that is not entirely true and an opinion, but it is mine and I stay away from that stuff.

We all have a fit range, I can ride 55cm to 58cm and be happy, like I said, 56cm is my sweet spot.

As to stand over, for cross and mtbs more is better, for street and commuting less can be acceptable but for me, I get nervous about any bike that I do not have approximately 2 inches clear in my stocking feet and more does not hurt my feelings. I have a performance and racing background so obviously that is influencing my smaller (in my fit range) is better than larger thinking.