The General Rule
The general rule is the shortest paddle that allows you to comfortable reach the water is best. In the middle of your stroke, your grip hand should be at the height of your nose. The throat of the paddle (where the blade meets the shaft) should be at the water line.
Measuring the length of your torso is a good guide to the size of paddle you should be using…
Measure Your Torso
Sit up straight on a flat chair. Measure the distance from the surface of the chair to your nose. That distance is your torso length. Then follow this chart to find the right size paddle for you:
An easy way to measure in the field is to place the grip of your paddle between your legs while sitting. The throat of the paddle is where the blade meets the shaft.
On a straight-shaft paddle, the throat should be at your hairline. With a bent-shaft paddle, the throat should be at your nose.
NOTE: Your size for a bent-shaft paddle will be 4 inches shorter than for a straight shaft paddle due to the shorter paddle stroke.