I think you can use any size. I haven't done any yet (want to, though!). I'm sure I read on here that someone used a larger block for the one that will be the center block because she wanted it bigger, and it worked out well too. So I'm thinking that as long as the corner blocks are all the same and the center doesn't exceed the length of what will be the finished block, you could do it any size you choose. But again, I haven't played with it yet, so I'm not certain. :)