With pet quilts, I would make the blocks large (fewer seams = stronger).
Do you have the fabric already, or are you going to buy something after figuring out the block sizes?
If I were working from stash, I would make the blocks whatever size was convenient given the fabrics I had on hand and the cutting board I was using. So say an 18 inch block, which would make each piece of the 9 patch = 6.5 inches cutting size.
It would be about 2 by 3 blocks, plus border, which would make it your target size.
More often than not, my block sizes are dictated by what size strip is easy to cut on the ruler that comes to hand (true confession).
RST