add 2.5 inches to the door size for width and 1.5 for height. if you buy a pre-hung door (recommended for DIYer) the instructions with the door give the recommended size of the rough opening. also, the manufacturers literature normally will give the rough openings they recommend for each size of their product.
For interior doors you add 2" to the width & 3 inches to the height.
Typically an interior door has a 3/4 jamb set, that will take 3/4 each side then you'll keep the hinge side to the frame and shim out the remaining 1/2 inch to the latch side.
The height also has the 3/4 then you'll have a finish floor and possibly a carpet. You usually want to have around a 1/2 inch to a fin fl. such as hardwood, but in a bath you may want more for the tile & a possible mat. So the 3" will give you that room, and hopefully you'll have the final design by then to know where to set the door bottom without having to cut it.
You also have casing so even if you drop the height an 1" or more you can still cover it by the typical 2 1/4 casing.