There is no specific description of how a jacket should fit in the dress instructions, but, unless things have changed in the 20 plus years since I had to refer to them to support my argument to get issued a proper fitting uniform (when we were being re-kitted for the DEU), you may find such information in "supply instructions". I have no idea what the specific reference would be now.
If my failing memory serves me, the supply instruction (which dealt with the fitting of uniforms for issue) stated that the (man's) tunic is to at least reach the "gluteal fold" (the crease that defines the bottom of the buttocks and the top of the thighs) when standing naturally. In other words your tunic should act like a good NDHQ staff officer, it should completely cover your a**.