Most prehungs have a 3/4" jamb around them. So I would add 2" to the door size to allow for shimming. For example a 30" x 80 " door I would rough at 82" x 32".. If you are rouughing it yourself make sure you square, and plumb the opening.
Every carpenter I ever worked with does it the same way: Add 2 1/2" to width and height of door itself for rough. 2'6" 6'8" door R.O.= 2'8 1/2" X6'10 1/2" If you are working from a sub floor set the door 1 1/4" from floor if floor is already finished try for 1/2 space under unless floor is way out of level in a way that would interfere with the door's swing.
As stated, mind the finish floor height. I would go 83" from there unless you are using some tiny casing. Otherwise you'll have to cut off the leg jamb ears that protrude above the head jamb. Just trying to save ya a step. With new prehungs, 2" over the door width should give enough wiggle room in the RO. You don't want to use too many shims, two opposing ones is ideal. Good luck.