Some PE teachers would use a boy’s own plimsole and other teachers would sometimes ask for boys in the class to offer their plimsolls when a boy was called to the front and, for whatever reason, the teacher did not have his own slipper. There was usually a chorus of boys offering their plimsolls and it was often the one from the biggest boy – who was likely to have the largest shoe size – that was accepted.
However, most teachers kept their own plimsoles, in their desk drawers or cases. These did vary in size but were all adult sized. I don’t think the effectiveness varied just in proportion to the plimsole’s size – the skill and experience of the teacher and the force used were probably more important.