If you plan to use a single block size then the size is personal preference. Many of the swaps are 12"
If you want to use multiple sizes in the same quilt, then it is easier if the size of the smaller blocks can be divided into the biggest block. So if your big block is 12" then you can work with 3" - 4" - 6" for sure even a 9" can be used when paired with a 3."