28 inch is all you can get in that. Any door needs a minimum of 1 1/2 more than the door size.
The rough opening width for a 24" door is 26-1/2" The rough opening height for a 24" door is 26-5/8" Rough openings are 2" wider and 2-1/2" taller than the actual door. This allows room for the door frame and adjustment.
The rough opening of an interior door is two inches wider than the width of the door and two inches higher than its height. Thus your rough opening width is 38". A typical interior door has a height of 80" which would require a rough opening height of 82" but check your door to ensure that is theheight.
R.O. is normally 2-2 1/2 inches greater than door size.
4'1 1/2" by 4' 1 1/2"
Remember 36" is the width of you door slab. If you can measure the door you want to frame in then take the measurement and add 3/4" to it so you can still have room on the side to shim it properly. If you don't have the door but know which one you want, go to the store and the door will normally have the rough opening size on the label.
standard height door (80" tall) 2' 6" wide door. Rough opening width is oversized 2"
You are supposed to measure the "rough" opening. This is distance from the studs that line the door opening. (not from the door frame itself) Top to bottom then from side to side. If you already have a door there and are just replacing it then you can just measure the door and get one of the same size. Good luck.
Rough opening requirements vary by door manufacturer. It would be best to check with the lumberyard where you plan to purchase the door, but if you've got to get it framed now, make it 38-1/2" wide, and 6'-11" high. You can always build it in later if necessary.
32" is typical.
If you are installing a residential type door and trim, add 2.5 inches to your door size (height and width) AND another 1.5 inches so you can screw a 2 x 4 around the inside of your rough opening to nail your door casing to. (screw the 2 x 4 in before sheet rock)
Add 2 1/2" to both dementions. for a 36" x 6'8" door you would frame 38 1/2 x 82 1/2".
Add 2.5 inches each direction to the size of the door. So, a 3 0 x 6 8 would be 38 1/2" x 82 1/2".