If you want to use 1/2 inch seam allowances -
Take the finished size and add 1 inch both ways.
Examples -
2 x 6 finished size strip - cut 3 x 7
4 x 4 finished size square - cut 5 x 5
6 x 6 finished size square - cut 7 x 7
If you want a smaller than 6x6 finished square, consider going to a 4.5 (4-1/2) inch square. That divides easily by 3.
4-1/2 divided by 3 = 1.5 (1-1/2)
If you want to go that route with the 1/2 inch seam allowances - cut your strips 5-1/2 x 2-1/2 (5.5 x 2.5) inches