The formula for each piece is this: Subtract 1/2" from each dimension. Divide each dimension by 2. Add 1/2" to each dimension.
Example: 8 1/2" x 4 1/2" rectangle to be made half size.
8 1/2" - 1/2" = 8. Divide by 2 = 4. Add the 1/2" back = 4 1/2"
4 1/2" - 1/2" = 4. Divide by 2 = 2. Add the 1/2" back = 2 1/2".