How do I find the correct finger size?
If she wears a dress ring on the third finger of her left hand, slip it on to your little finger and make a mental note of where it slides down to. You can then take the size from that.
Generally if a ring needs to be altered again there is no problem and no appearance of where it has been altered. Thurlow Champness PORTFOLIO of Fine Diamonds offers a free altering service so that you can be sure the ring will fit when you give it to her.