Another thing to consider if you are talking about the wedding band, is what you want as well. The engagement ring is more about her alone (aside from wanting to get one that would go well with the wedding band eventually). If you want to have wedding bands that match, you also need to consider what you want to wear. Some couples don’t wear matching wedding bands, but it’s something to think about.
With the engagement ring, like the others said, it really depends on what she likes. Have you ever been in a jewelry store with her before? If so, did you notice what types of things she was looking at? If not, maybe you could take her to look at something else (maybe a necklace or something) and then see if she happens to look at the other things as well while there. Pay attention to what she looks at. As for figuring out her size, if you don’t know, you could ask flat out. If you don’t want to ask flat out, see if you can get a hold of one of her rings, try it on your fingers. If it fits one of your fingers perfectly, you could go by that. If it doesn’t, try to remember how far down it would fit on the finger you tried to get it on. Worst case scenario, you might have to get it sized after you pop the question.
For gold, the lower the K, the stronger the metal and the less pure it is (it’s stronger because it is less pure, the other metals in it are what make it stronger). The higher the number, the more pure it is, but also the softer it is. 24K is pure gold (and the highest K).
I’d shop for the wedding bands together once you are closer to the wedding so that you can both get what you like.