If you are making a 10" block, the squares would be 2.5" and the squares to start your half square triangles are 3" (technically, 2 7/8", but I always make them big and square up ti size later). If it is 15"' then 3.5" and 4"' etc. if you are trying to use 2.5" squares, cut them apart and sew them back together, they will be too small because of the seam allowance.