Suzanne,
I used 5 inch blocks. I have a friend who owns a fabric store and she keeps me in fabric for charity projects. Using the 5 in. for making the block it starts out at 15 inches. After cutting and resewing and squareing up the block it will make a 12 1/2 in. finished blk So you lose about 3 inches. I would guess a 12 inch starting block would finish around 9 1/2 inches . Hope this helps.
Keep Quilting
jayna