I use that exact lens on my old Toyo Field camera that takes 4X4 Graflex lens boards. The opening in the board seems to be about 26 mm. That's measured as well as I can with a ruler.

The shutter is not likely to be a Copal though, more likely a Compur. I've seen three or four of these lenses and they have all been (mine included) on Compur's. I guess there is no reason why this lens could not be retro fitted to a Copal. If there's a pain in the butt with this vintage Compur's, it's that there is no lever for opening the lens to focus. You have to put the lens on B, then lock your cable release to keep the shutter open.

Mine has no markings indicating what size of shutter it would be (compared to a Copal) but I can say that it's pretty small.