My favourite are 65 Krt. Also they should be at least 27cm in diameter.
Well, it mostly depends on her stature. If she’s petite, then one carat will look huge and gaughty. If she’s 6 feet tall, then they might be too small.
But, on an average person, something just shy of a carat is good. Also, once you go over 1 carat, the prices start to go through the roof. Shoot for something between 3/4 and 1 carat, and nothing less than an si2 clarity, with nothing less than a G color. If you have to choose between size and quality, go for the quality. Nothing is worse than big diamonds that look fake or cheap.
Try for some very very reasonable loose diamond prices, or check with a local (non-chain) jeweler.