Sorry, I hadn't read the responses before I chimed in about the serving sizes .
Costumeczar, I don't have a 7" square pan (and can't foresee a need for one) so the smallest I'd go would be 4" on 8" and I'd charge them for 48 servings.
I have one! I use it often, but not half as much as I use my 7" round. I actually bought a second set on Monday. I have pans in 1" increments so that I don't have to sell someone way more cake than what they need, I can scale it back for their count. The place I used to work had 9" pans, but that was their only odd size, and they would over shoot the serving count by 50-100 quite often. Which means more money for them, but quite often I hear people talk about how they don't want that much cake, so they chose me over them ;-)