Are you going to provide the inserts or just do the covers? In the stores here, inserts only come in a few standard sizes, like 16"x16" and 18"x18". So if I were buying a pillow cover from you, I would want it to match a standard insert size.
Of course, if you're going to stuff them, then the sky is the limit. I would buy little ones for my daughter's bed (like 12"x12") and larger ones for the couch (16"x16"). I have a couple of knitted pillow covers and I love them!