While sitting down, ideally on a bench, rest the canoe paddle handle on the seat beside you or between your knees. With your back straight looking straight ahead your chin should be about 6 inches below where the blade begins. This is a general rule of thumb we use which measures the length of the canoe paddle against the length of your torso.
For kayak paddles the length of paddle needed depends on both your height and width of kayak. Generally the taller you are and the wider your boat the longer the paddle should be.