I think it is what you like personally and the pattern that you are using. Currently I am using 14 1/2 finished blocks in the quilt I am making my brother for Christmas, just because I wanted large blocks and I started out with a 15", but after the quilting was done on the block and I trimmed it, the first block ended up 14 1/2, so I made all of them that size. The quilt is reversible, so each block is quilted before they are put together. When I get it finished I'll post pics. Hopefully I will get it finished this week. Started on it after our Fall Fair in October and have really been working on it. But I will have it finished by Christmas.