I would stay with 6 1/2 inches, finished.. which means you would cut your piece 7 inches
Then the 4 patches would be cut 3 3/4 to finish 3 1/4 each, giving you a 6 1/2 finished block. or 7 inch unfinshed block, same as your big block.
What ever you do, here is how you figure it.
take the finished size and add 1/2 inch for cutting size of the big block.
take the finished size of the big block and divide by 2 to get finished size of four patch squares. add 1/2 inch to that and you get your cutting size for the 4 patch pieces...