Grip circumference
Handle thickness. Most paddles ship 4.125–4.25 inches. You can build up, but you can never shave down.
Too big, and the grip locks up your wrist and tires your hand. Too small, and you death-grip it. A tennis-elbow contributor. Most paddles ship between 4.125" and 4.25".
Overgrips add roughly 1/16" each, so when you are between sizes, buy the smaller grip and build up. One reviewer-consensus warning: stated and shipped sizes do not always match. If you are sensitive to it, check before you buy.
See also:Handle length ·Paddle shapes