This can be a lengthly discussion. All blocks are not easily created in all sizes. So, using this particular block as an example..
It is a 16 patch block, 4 rows of 4 blocks.
If you want to make this as a 16 inch block, divide 16 inches by 4 (number of patches in a row) = 4 inches finished.
For the square pieces, add 1/2 inch for seam allowance. So, cut them 4 1/2 inches square.
For the half square triangles, add 7/8 inch to the finished size, or 4 7/8, then cut in half from corner to corner (on the diagonal)
Hope this helps