5/6?
I don't know. I just know that when we see swirls which are larger than 3/4", the ID's guessed are almost always Alley, Jabo or Vacor. Larger is surprising to me for a Champion. Every now and then I ask what the latest info is on the size range we know about from the different makers but just knowing the max doesn't tell how often the max is reached.
Just now checked in American Machine-Made Marbles and see that I shouldn't be so shocked at larger transparent marbles -- but larger non-transparents might still have the capacity to blow my wee mind.