I have them and one is 7 1/2 in and the other is 9 1/2 inchs.
The small one is for the blocks and the batting. The large is for the backing that pulls to the front and you sew down for the sashing.
You put all three layers togather at the same time. You are quilting the block as you go using strips for the block design, then pulling the backing to the front of two blocks and sewing them togather then you sew each side of the backing over each block to make a sashing. Hard to explain but really simple.