remove the seam allowance measurement from the size of each piece, double the size-then add the seam allowance back on---an example-- if you want to double a piece that measures 1 1/2" x 4" you remove the seam allowance (1/2") so you have 1" x 3 1/2"...double these measurements---= 2" x 7"; add the seam allowance back on--= 2 1/2" x 7 1/2"
works most of the time to increase the size for pieced blocks....if you don't remove the seam allowance before increasing the measurements don't come out right (you would have 3"x8"- using the above example- your pieces would be too big to make the block- things would not measure right)