Of course, the slipper also had a strong deterrent effect when used by the PE teacher. Many would rather use a larger slipper that would whack both cheeks each time, than use a smaller one, leaving a small red mark where it was applied to just one buttock at a time. The reason being, the boy would have to shower and so the rest would see the marks. A completely red bottom was more of a deterrent to others than one that just had a small red mark on each cheek.
I do think the tales of size 13 adult slippers being used may not be entirely true. These would be very heavy and more likely to bruise. On a more practical point, the heel would be hard for the teacher to hold, unless, like his feet, he had a large hand.