The SPOOL is 3.715" wide. I'd guess the film itself would have to be a tiny bit narrower than that so it doesn't jam between the flanges.
Originally Posted by anikin
I just measured some actual 122 film stock, and it came out as 3.614", more or less. I don't know whether this is factory VP122 or some of that 105mm Ortho Copy microfiche that I cut down myself; the edge markings only say "KODAK SAFETY FILM" with no frame numbers. 3-5/8 is exactly 3.625", so this would be pretty close.
edit to add: when you say you're making 122 film, I don't suppose you're going to be selling it with backing paper and/or spools?
Come to think of it, there'd likely be a great market for just the backing paper. Heck I'd even buy a wide roll of plain backing paper myself and slit it down as I need it; spools I can make myself if I have to. Any possibility of making the backing paper available for sale?