I haven't been into watches for all that long but the first thing I noticed was how stupidly MASSIVE most modern watches are. Most are way too big for my apparently average 7 inch wrists. So in terms of standards, I cap any potential purchases at 38mm as this is the largest case size I'm comfortable with. The only other 'standard' is that if a watch is on a bracelet, the bracelet does not taper to less than 16mm as anything smaller looks feminine to me. Unfortunately this rules out many of the more affordable Rolex models.
I would make an exception to either of these standards if a watch feels and looked right when trying it on in the flesh (the 40" Submariner actually works for me).