Mine is 5/16 I believe. Actually the thread conversion kit is separate from the driver. All I have is a 5/16 conversion kit. If you're willing to go bigger, or buy a different (McMaster-Carr) conversion kit, I have one.
I have a home made one. Just a square tube with two holes. You put a grade 8 bolt through one of the holes, stack some washers, and hit with an impact driver.