I guess I should ask if you are going to go to a custom framing shop or not. If so, try to decide before hand how much wall space you want to cover. An 8x10 photo matted with 2" around each side and a frame that is about an inch on all sides makes the overall picture about 12x14. Everything, however, expands as the print gets larger. The larger the print, the more size is needed on each side of the mat and frame. It's really a issue of balance.
Print rough drafts in the most common sizes and take them with you to see how they will look in different size frames. Some shots will look better small, others scream to be enlarged.
My 1.414 cents worth.