Check out Quilt.com as a reference for all kinds of blocks, all different sizes. If you are designing the block yourself, graph paper is a good way to go. Also think in terms of the number of units in the block. For instance, a 4 patch that finishes to 4 inches square needs 4 squares at 2.5 inches each. A 9 patch isn't going to be able to be easily divisible by even numbers, as there are 3 squares across. I think in terms of there being "even" blocks and "odd" blocks in terms of size, so there are 4, 6, 8, 10 12, etc and there are 3, 9, etc. A 12 in block is a nice size to work from because you can make it in either "even" numbered units or in "odd".