trick the girl la.. take her to the shop.. get her to try the rings..
ask her which design that she likes..

or if got magazine got ring ad or something or if got catalogue, then ask laa.. which one she likes..

or, can ask her sister or mother or best friend what kind of design she likes..

size can pinjam one of her rings2 diam2.. get her sister/mum/best friend to help..
if not, like what others said, size can be adjusted later..