Oh no, you have to chose it yourself! It's part of the whole "proposing" thing in my opinion, and shows you have made a huge effort!. Can you maybe try to get hold of one of her rings that she never wears, and get the size that way? Also, make sure you do your research and ask her best friend, family...etc...what style she would like best, as long as you don't mind them knowing and they won't spill the beans! If in doubt, go for a "classic" cut, something timeless, yet elegant.
Congratulations!!!