If you use a 12" (finished) block, you can also use 4", 6" and 8" (finished) blocks. The way you put them together just has to add up to 12". So, 4-6" blocks = 1-12" block; 2-8" plus 2-4" blocks = 1-12 inch block; and 9-4" blocks - 1-12" block.
Or you can decide what your row measurements will be, and then shuffle blocks around until they come out right.
Graph paper and a design wall will be your best friends!
I hope you post pics when you are done.