AnnaMarie, get a pad of 1/4 rule graph paper and chart your quilt out using the square to equal certain sizes. If you want 6 in finished block, 1 square on the paper would = 2 inches.
you don't have to be exact...draw lines down and across to represent your 6 in blocks. Then count the block you would need to make whatever size you want. Remember to add in the seam allowances.