The first block is based on a 3x3 grid. When you make the block finish at 12", each piece in the grid finishes at 4". If you change to a 20" finished block, each block must finish at 20/3 = 6.66666". This unit is much harder to measure. Any block based on a 3x3 grid is easier to finish at a size that divides by 3 easily. This doesn't mean it has to be 3 times an integer (3,6,9,12,15,18,21,24,etc.). It could also be in half steps (1.5, 3, 4.5, 6, 7.5, 9, 10.5, 12, 13.5, etc.). So, short answer - I wouldn't try for a 20" block, but would pick something close that divides easily, probably the 21" block.