So if you've been engagement ring shopping, visited your jeweler, you've probably seen one of these. It's a ring-sizer. So, simply enough, there are numbers on each one of these little rings. Put it on your finger, make sure it fits snugly. It should be a little tough to get on, a little tough to get off. Mine is a 4. Try a half-size up or down and see what's most comfortable for you. If your girlfriend has not had her finger sized, there's a couple ways you can get her size. Well, you can ask friends and family, see if anybody knows.
Take a ring out of her jewelry box that she wears on her left finger. Keep in mind, a right hand sometimes is bigger than our left if you're right dominant and vice versa. You could take one of her rings, draw it on a piece of paper to get the diameter. Maybe put it on your finger, take a ring and just put it on your pinky, see how far it goes down, then you can get kind of a rough measurement of what her finger size is.
If none of those above work, well then it's safe to go with an average size around 5 or 6, just make sure you buy a ring that you're jeweler says that can easily be sized up or down, so that way you'll be able to select a ring with the right size.