If I understand you correctly, you want to add a strip to all 4 sides of a 6.5" block to make it a 12.5" block (I'm assuming 6.5 and 12.5 are the unfinished sizes).
12.5-6.5 = 6 So to make it even all the way around, you would divide the additional 6 inches by 2, which gives you 3Now you have to add the two 1/4" seam allowances back in so if you cut 3.5" strips and sew them to each side of the 6.5" block, you will wind up with a 12.5" one.