I think we have the same wheel. It's been a few years and eventually I became confused who made it. Now it came back to me it has to be Max Daddy as well.
I bought it used but in perfect condition. It was a super deal I realize, and I hope I want make you upset L0L I paid $440 plus shipping. I think the guy bought it new for $1200. Years back, no idea how much would they be now and if available.
It's 18 5.5". I'm using radials front and back: Avon AV72 Cobra Rear Tire Blackwall 180/55ZR18 74W and Avon AV65 Storm 3D X-M Front Tire 110/80ZR18.
I confirm, this is as wide as I could go with everything stock. To go wider I'd need to notch the swing arm and modify or move the rear brake caliper stay.
But I want be going any wider, as this feels just right. It took a bit of time to get used to riding on wider radials switching from ply (I had Shinko 170). The bike felt softer yet because of the width and the tire profile it seemed like it wanted to stay upright when standing still or slow speed, which was weird.
By the way, there is a tire size calculator online and you can compare how different tire sizes would change the speed readings. With a 17" wheel/tire there would be difference, with 18" it's negligible.