I highly doubt it is either one of those. I haven't actually removed mine but I would bet it would be something like a 14mm Just get a socket set that goes up to 19mm and you should be covered.
Stick your Mellon under the truck and you will see there are four bolts that you will need to take off. Follow the brace and you will see them. If I remember it is a 17mm socket you may want to spray them with some WD or bpblaster yo help loosen em up! Good luck bro!!
seriously,...jack the rear end up...slide underneath with socket set, find all 4 points of the x-brace. you will find a bolt at each end (4) take the bolts out, and roll out with it
glad you got it off and painted. I had mine painted when I had my rear end damaged fixed. I think it really sets the X off to have it painted. good job.