Congratulations on your first show, Ann!
When I don't want to break the bank, I go with:
- 5x7 matted to 8x10
- 8x10 matted to 11x14
- 11x14 matted to 16x20
(They don't have to be matted; sometimes I slip them into an archival bag with a sheet of acid-free backing board, from http://clearbags.com )
Then, I'll add in any larger, or framed, or leftover work from the last show. (I know this doesn't apply for you . . . yet! ;-)
It's important to have a range of prices -- something for everyone -- and buyers really do like small-ish prints. For instance, in more than ten years I've sold two 8x10 prints online. In the real world, I've sold more of them then I could ever remember!
If money is no object, or I have many months to prepare, I like to have four-to-six larger (above 16x20, or 20" square) framed, metal, or acrylic pieces to show. So much of this depends on your own budget and transportation, plus the type, size, and expected traffic for the show.