The largest FAT frames produced were the 23" Fats and Wickeds in the late 80's and early 90's while I was there. These were produced in very small numbers compared to the smaller frames. I'm not aware of anything larger produced and it would have been custom. The 23's were sized for riders up to 6'-6" but I don't think I've ever seen one for sale here or ebay. I would say that there were less than 50 produced overall of both models combined in the eight years I worked there, but that's a guess at best. I have no knowledge of what was built after Somerville closed.
The Yo Eddy was built in Large which was a bit bigger than the 21" Wickeds and Fats but has the sloping top tube like all Yo's have.
Good luck
Scott