How do I get my girlfriend's ring size?

Why not buy couple rings first at places like Couple Lab? 😝

measure it while she is sleeping? use a string. Just to get an approximate. Or if she has any of her own rings? U can take measurements/photos(on a measuring tape) of that. With an approximate the jewelsmith will usually size it just slightly bigger. So try to find the best approximate u can.

As Yang Teng mentioned, u can get it resized afterwards.