Measure wall to wall and type it in. The short walls are the ones the ceiling comes down to; the peak is over the middle.
Measure it and type it in.
Paint
Boards run front to back, six inches wide, so the pattern gives you a sense of scale.
Solid. The house is 22 ft along the street by 28 ft deep, off the city assessor's own building sketch. Take off the outside walls and the inside width across the front is about 21 ft. That is the wall the window is in.
Solid. Built 1927, one and a half storeys, 924 sq ft finished. The ground floor is 616, so about 308 is counted upstairs.
A guess. 13 ft 6 in front to back is just half the inside depth. No record says where the wall behind this room actually is. Measure that first.
From the photos. The ceiling is slope, flat, slope, not a peak. The flat part in the middle is a suspended tile ceiling and the slopes on both sides are covered in comic posters. That flat band is why a tall thing can stand down the middle of the room.
From the photos. There is a storage nook cut into one of the short walls, deep enough to stand a bookcase and guitars in, with the roof sloping down at the back of it.
Still a guess. The short walls at 4 ft, the flat ceiling at 8 ft, and the flat part 6 ft wide. Those three change what fits more than anything else here.
The free ruler. That tile ceiling is a two foot grid. Counting tiles across the flat part and along the room gives both numbers exactly, without a tape measure.