If you start with 7" squares ... sew the two diagonal lines and cut apart ... you have two HSTs that are NOT 7" squares. You lose size with the seams.
You will have very little to trim off to make them into tidy 6-1/2" HSTs.
The rule of thumb is you cut your starting squares 7/8" larger than the finished size of the HSTs you want. Those 6-1/2" HSTs are going to finish at 6" once sewn into place, so the extra 1/8" just helps give you perfection!