My question is, what should I know, or should have known, about the sizing or 'geometry' of a bike?
In reply to my question about buying, people talked about the wheel size etc.; but not about the frame size, or the fork angle.
In the question about test-riding, people talked about aspects of the 'fit' which I think can be adjusted (i.e. the height and front/back position of the seat, and the height of the handle-bar) ... and which are documented elsewhere ... and which are therefore maybe superficial and less important for the purposes of this question.
But, what about the geometry which can't be adjusted: the distance between the wheels, for example?
On a bike which I've just bought, I've noticed for example (since buying it) that the front wheel is rather closer to my foot than I am used to; I expect that's just one example (of a geometry/metric which as a novice it hadn't occured to me to think about).
This Wikipedia article "Frame geometry" introduces the vocabulary but without much detail. Googling reveals mostly short articles too, more ad-bait than detailed/informative.
Given that I'm asking for details, a kind of 'science' question, this question is probably too long to answer here. What I might be asking for then is a link to a suitable introduction/reference (online, or a book...).
Alternatively you might (I don't know) be able to give a short/summary answer along of, "The most important measurements to look at are A, B, and C, which have the following effects... Taken together these few most important measurements account for 90% of the effects of geometry on fit/performance/handling/comfort."
Or, is this a topic that people don't learn about by reading, and which can only come from extensive experience of riding multiple bikes? How did you come by your understanding of geometry? Can you look at two bikes of a similar type and say, "Oh, those have different shapes/measurements" and understand what that (difference in measurements) means or implies?
PS.: interested in 'ordinary' frames: not frames with a suspension (so, not frames for off-road/all-terrain/jumping); not recumbent; not track/racing; not exotic/expensive materials.