Start with whether you carry it or it stays put. If you show work on the go, the Easel Binder with Handle carries like a briefcase and opens into an easel — rigid 3mm board, a spring handle, a locking multiring, and an elastic closure to keep pages secure in transit; it's refillable, in 8.5"x11", 9.5"x12.5", and 11"x14". If it lives on a desk or counter, the streamlined Professional Display Easel has no handle in the way and presents cleanly upright — leather-grain outside, sand-grain inside, refillable, in 8.5"x11", 11"x14", and 14"x17". And if you want a finished book you can also hand someone to page through, the Bound Easel Portfolio Book is a hardboard book that lays flat for close viewing and stands as an easel — in portrait and landscape at 8.5"x11" and 11"x14" plus a 14"x17", with 10 permanent display pages.
Then match the size to your paper format: each is built to hold that standard size of work, so pick the size that fits your prints directly.