At a mass slippering for messing about in the changing rooms about 8 of us were lined up in front of the rest of the boys. The P.E teacher asked us in turn our shoe size and told the boy with the largest to take a plimsole off. He then whacked us with that one. I dont think it was bigger than about a size 8. This was in 1973 aged around 13 at the time.
When talking about size 13s in slippers bear in mind that (at least in the 1950s) there were child sizes in footwear going up to 13 so if a size 11 was used it could be quite a small one.