2"
BTW. a 1.25" shaft collar available at a hardware store makes a perfect, low cost, 2" to 1.25" adapter. It even comes with a threaded whole and setscrew. Replace the setscrew with a nylon thumbscrew and paint it flat black. It is heavier than the aluminum adapter that came with JMI focuser, but perfectly useful.
A shaft collar is a ring of metal (cylinder with a concentric hole) in this case 2" O.D., 1.25" I.D., and about 0.75" thick. This one is designed to go on a 1.25" shaft. For locals, I got one at Orchard Supply Hardware.
Chuck