A good rule of thumb for a great fit is to select a belt that is at least 2 inches larger than your pants' size. If your pant size is an odd number, increase the belt size by 3 inches, e.g. a size 33 pant should wear a size 36 belt.
This is what we find works for the majority of our customers. Please note that pants size varies by brand (what one brand calls a 32 can be different from what another brand says is a 32) so we suggest looking at a couple of different pairs of pants and using the one with the largest size that fits you.