I've participated in a few round robins. Usually, we've started with a center block of our choice, between 10 and 12" finished. Then the height of the rounds is prescribed, usually between 3 and 6". That still ends up with a fairly good-sized quilt in the end, so the last couple of rounds were given 2 months to finish. Some of these quilts have been gorgeous. Most of the ones I've been in have had no other rules except for size, but one that was fun suggested incorporating different things each month - squares, triangles, half-square triangles, hour glass blocks, etc. Personally, I don't like row robins as much because they end up with a rather skinny quilt (36" or so wide).