any reputable jeweler will tell you to pick a size above (1 or 2 sizes) what she would need, just to make sure she will be able to put it on. even if you provide them with the exact size, they won't adjust it for you until she tried it.
I had stolen her 4th finger ring of the other hands, brought it to them, and they still wouldn't do it.
I think it makes sense.
My girlfriend is a control freak as well and is picky about jewelry but she loved the fact that I spent so much time picking the right size, clarity, setting, etc...
She never asked to change anything, she doesn't even want a larger diamond since she has small hands and feel it wouldn't match.
Congrats.