No help?
Thought someone would speak up by now. I'd be guessing at 1/4" which is what a lot of the 1" shafts took. But I would call Hunter as the next step. If the boat is out of the water you could take out what is already in there and measure it. Or you could take out all the rings, put one back in and then use "Dripless". Dripless goes in between two rings of regular packing and old rings would work fine. After two seasons, no adjusting and no drips. See Link.