The two slipperings that I received at school were from a plimsoll.
On the first occasion the teacher used the boy’s plimsoll on the front desk, I’d guess a size five.
For the second slippering, the same teacher found out who wore the largest plimsoll, size nine came out on top and that was used.
My wife’s headmistress used one of her old mule slippers to punish her with, having seen her I would estimate her shoe size was seven or eight.
But regardless of size, we both found it uncomfortable sitting down afterwards, as both teachers were very experienced in the art of slippering.