If you go to member swaps there are quiet a few round robins going currently: bluebirds, hummingbirds, blue jays and they are all medallion style round robins. I am running one here also called the road runners round robin and we are almost finished. Tlambing has been kind enough to post one of the group shots along the way so you can see how a center starts and progresses throughout the round robin. There are only 9 pages so its easy to see the whole process. Your center can be anything you want. I would say make it easily divisible by 3.5 or 6.5 (I prefer 4.5 and 6.5) so you wont have to be too creative on your corners. some designs fit easily into a round and others run short where you will get adventurous and perhaps put a block in the center to break it up so it hits the corners nicely if you have an odd sized center. Does this make sense? trying to explain this better.
http://www.quiltingboard.com/picture...a-t227970.html