As regards the size of shoe used, it did change from teacher to teacher. Those that used it in the classroom, tended to have one that they kept in the desk. These slippers were mainy about size 9, as this was thought ideal for whacking most of the most receptive part of the boys bottom each time. These slippers were given names like “Charlie”, “Fred” or “Mr Sting”.
I always found that PE staff made the greatest use of the slipper. In most cases, this was a gym shoe that was in lost property or one borrowed from another boy. If this was a junior school, the chances were that the slipper was fairly small, maybe as small as a 4 OR 5.