27mm, normal thread. Takes a bit of a grunt to break the nut loose, but then it will spin off easily.
Yes, you need to have the bike in first gear, rear brake applied fully, and the chain has to be on as well.
If you don't have any help available, here's what worked for me
1. 1/2" drive ratchet handle with breaker bar slipped over it - get this settled nicely on the nut.
2. Use a broomstick or the like to push down on the rear brake pedal while pulling up on the breaker bar to break loose the nut.
When you hear a snap, you can stop pulling on the ratchet - that was the sound of the nut breaking loose, and you should almost be able to just spin it off by hand now - I could with mine.
Reverse procedure when tightening the nut back down with your torque wrench set to on the proper spec.