"When does the diameter of the hole stop it being a cup and make it something else?"

Dunno. A bullaun can be anything over around 10cm across x 5cm deep, although that would be a very small one. If one that size was with rock art then I'd class it as a big cup, if it was on its own then I'd tend to say it was a bullaun.

As far as I know, no bullauns that appear with rock art.