How big do I frame the door entrance for a closet sliding door
although every situation is different, generally when you frame the opening, add:

horizontal
3/4" to each side for the jamb (or whatever the jamb thickness is)
1/2" to each side for the gap for shimming the jamb
1/4" extra for bifold or french doors

vertical
3/4" to the height for the jamb (or whatever the jamb thickness is)
1/2" for the shimming at the top
3/8" to the bottom for the gap between door and floor (we usually do 1/2", or 3/4" if there are carpets or matts being used in winter

so for a 72 x 80 bypass door, your RO will be approx 75 x 82.  since its a bypass door and the floor could be out of level, i would do 75 x 82 1/2.  

remember too that trim will cover a lot of stuff up, particularly now when most casings are in the 4" or wider range.