[freakshow]

You need to go in and see them in person. When you're doing ring shopping/research, you can spend hours getting into the specs and numbers like you're comparing engine horsepower.

Go into a jeweller, and have them show you different carat sizes, different colors, what inclusions of varying sizes look like under and not under a loupe.
Once you see it it person, you'll get a better idea of the difference. Often times you can really only tell the difference between, for example, a F and a G if they're side by side.

FWIW, my wife is 5'5, ring size 6. I got her a 1.14ct round (10 years ago), and IMO it doesn't look too big at all. It's just about the right size. She could go to 1.5 and I still think it wouldn't look ridiculous.