As mentioned, it's the posts/metal that irritate the ear. Unless you've got some major cash, I doubt you'd buy diamonds that would be heavy enough to cause problems. Just make sure they're set in white/yellow gold or platinum.
The size really depends on how much money you're wanting to spend and the quality of the diamonds. I'd say get the biggest you can afford.
But I'd try and get at least 1/4 - 1/2 carat per ear. And remember - if they tell you they're 1/2 carat, that probably means each earring is 1/4 carat. Be sure to ask the size of each individual earring.
I wouldn't do dangling or colored stones. If you want something that's really pretty, get a different cut on the diamonds. I have a pair that are 3/4 carat each, platinum set, and they're radiant cut. They're gorgeous and I wear them almost everyday. I have a round pair but hardly ever put them on because I like the others so well. Have one of the sales girls try on some radiant, emerald, and princess cuts and see what you think.
This is what mine look like. The picture honestly doesn't do them justice.