What bike size do I need?

whacking this here largely so i can refer to it as a link , to save me the time on the seat post size / top tube debate during sales here and elsewhere.

the bikes behind are identical in geometry and set up. but could be built as old 40s/50s style oversized frame or compact frame with no change in the position of feet, bum and hands.

sure there maybe some differences (for eg. long seat post may give more flex - compact maybe lighter , the oversize might get you in the knackers (standover too high) or one may or may not subjectively more pleasing aesthetically , - but for position - no difference)

so vastly different ''frame sizes'' (seat tube length) can give the same 'top tube' (virtual) length, wheel base, steering angle

I don't quite get the point here.. the fact that two identical-sized bikes are identical in size? The fact that if you keep all the contact points in the same place all the contact points stay in the same place regardless of what green triangles you draw between them? I don't think many people would consider seat-tube length to be the sole defining criterion of 'frame size' - especially with compact frames - and even with a horizontal top-tube, top-tube length isn't an absolute measure of how 'long' a bike is, as demonstrated for example by this bike, which due to the small headtube would actually work out 'longer' than say 55cm square (depending on how high you have the stem, of course).

The fact still remains that if you have a position in mind, there is still a fairly limited range of frame sizes that will allow you to achieve that position.