In the Grassy Creek Mystery, clue 2, for making 2" finished QSTs, Bonnie Hunter writes: "If you were working with traditional rotary cutting math, you'd start with 3 1/4'' squares." (The clue then uses strips, not squares, to make elements of the QST, rather than quartering a triangle.)
If you try the experiment, or use math, you will see that cutting a square along both diagonals and sewing it back together yields a square that is 3/4" smaller than the original square (so the finished size is 1.25" smaller than the original square).