Don't worry about ring size. You can resize rings for about $30 at your local jeweler.
If she has small fingers like that then you are in good shape. Just get the larger size at first then resize later.
I'm a big fan of diamonds, as on my wedding band I have 9 of them
Some personal tastes of mine when searching for a quality diamond.
The color, to me, was most important. D is colorless, E and F are not really noticeable color to naked eye, and anything else is noticeable. If you are getting a gold band, you can move down into the HIJ color chart for diamonds because you won't notice the yellow as much. If you are going white gold, which I personally prefer looks wise, I wouldn't go any lower than F color.
Second, I'd go with cut. Kay/Shaw jewelers like to claim their Leo diamond is amazing because it has something like 102 facets in it. I wouldn't worry about the facets as much. To the naked eye, an 80 facet diamond is going to shine as much as a 100 facet. The important part of the cut is to make sure the dimensions are symmetrical. I prefer a shape just like this. ->
http://www.am-diamonds.com/images/anatomy_diamond.gif Third would be Carat size. If she has small fingers, I would go with something like a Solitaire Diamond (most sophisticated look imo,) 1/2 carat to 3/4 carat. The 1/2 carat on her finger will look like a full carat on a woman with larger fingers. Price is also a big factor in carat size.
Lastly, I look at clarity. In all honesty, i don't even worry with it. You very rarely can see clarity with the naked eye.
I bought my wife a 0.56 Carat, D colorless, 86 facet, VS1 clarity, solitaire engagement ring with white gold band. Her wedding ring is 5 diamonds, 0.54 carat combined, E color, SI2 clarity, and I don't recall the cuts.
Hope this helps. These are my opinions, so make sure you go see for yourself what you like. Hopefully there will be a congratulations in order for the future.
