For whatever reason, Icom doesn’t include mounting diameter in their specs. The included u-bolt will fit up to a max of 2.5″ pole. The mounting bracket is slotted up to 4″ so you could probably use a larger u-bolt of at least 3.5″. I’m not sure about minimum mounting, but I had it mounted to a piece of 1/2″ EMT shoved in the ground for field day one year and it was just fine.
Here are a couple of reference photos. The photos have a bit of perspective error to them because the tape measure isn’t flush with the objects.