Half square triangles - If cut from a square - the theoretical amount to the add to the finished size is 7/8 inch. I actually add an inch and trim the HST units down after sewing them.
(Example: Four inch finished size (two pieces make a square unit) - cut 4-7/8 inch squares
Quarter square triangles - if cut from a square - the theoretical amount to add to the finished size is 1-1/4 inch
(Example: Four inch finished size (four pieces in the square unit) - cut 5-1/4 inch squares.