So to satisfy my own curiosity I measured the 5 couplers I have at home with a set of calipers. Sizes ranged from 24.0mm (15/16-inch, or 30/32-inch) to 24.8mm (31/32-inch). With most being closer to 24.8mm than 24.0mm.
You are in luck in that I recently ordered from Brewhardare two of the...
Not all 1/2" couplers (at least the ones in my inventory) are all the same OD. I'd measure mine to let you get a sample of the range, but I'm not home right now.
Some sellers might have detailed dimensions, or willing to take measurements for you.