Carefully pry the wood casing away from the door frame from inside the room, using a small pry bar and hammer. Insert the bar between the wall and the casing, then tap the pry bar with the hammer to begin prying away the casing. Work in small increments along the length of each piece and remove, revealing the studs beneath.
Measure between the studs on each side of the door frame. Measure from the bottom, top and middle of the opening. Use the smallest of the three measurements as your door width. The studs are where the pre-hung door frame attaches to your opening.
Take measurements from the floor to the bottom of the door header board on both sides of the door opening and in the middle. Take the smallest measurement as the door height.
Measure the thickness of the wall. Pre-hung interior door frames come in two sizes to accommodate most interior walls.