This very much depends on your taste. I know people who find .25 ct each a great everyday size, while others prefer more than 1 ct per ear. Personally, I wear .5 ct per ear and I feel that's a good everyday size for me. Then again, I wouldn't mind going bigger! Stop by a local jeweler and hold different sizes up to see what look you like. Shrinkage is real, and can happen faster than you think, though.
Consider the setting, too. Some people like bezel set for everyday wear, while others prefer a 4 prong. I like a martini setting, and I prefer them in a white metal, even though I mostly wear yellow gold. With a white setting, it blends in with the diamonds and all you see is sparkle.
As for cut, I always prefer round for earrings. With any other cut, they can become twisted and look off-kilter. Additionally, an excellent cut in a round stone hides a multitude of sins and can make H or I stones look white (especially in earrings, I wouldn't pay the premium for colourless stones). Cut is absolutely the most important factor with any diamond. With round stones you can plug their angles into the HCA (Holloway Cut Adviser) to help determine if you are getting an ideally cut stone. Sometimes GIA excellent isn't quite as great as it could be. And I respectfully disagree about going shallow with a wide spread. More sparkle from an ideally cut stone will make stones appear bigger than their mm size alone.
