Sizing
When specifically speaking about rings, the key function the size of the ring plays is how much it constricts blood outflow from one’s penis. A smaller diameter will allow less blood out. Knowing your sizes in advance allows you to decide in advance how much you want to constrict (and how hard you want to get). This is all part of the balancing act - an act that is influenced by your mood and your intentions when you put the ring on.