If it's square pieces, you add 1/2" to the measurement for the seam allowance.
So if it's 4" finished, you cut a 4.5" square.
If it's a finished rectangle, say 3"x6", then you cut the rectangle that is 3.5" x 6.5".
That's not the block size, that's the component size.
Do you have a link to the picture of the block you're trying to make?