To choose the correct paddle size go as short as you can without sacrificing an efficient paddle stroke.    

Paddles are sized by overall length: tip to grip (or tip to tip).  However it is far more accurate to measure your correct size by using only the shaft (and grip in the case of canoe paddles), as the blade can vary greatly in length.  The part of the paddle that is out of the water is what needs to be measured correctly to match your body size.

There are a lot of factors that influence paddle size - the type of paddling you do, the width of your boat, the height of your seat... so please use the following as guidelines only.  You can play around with the sizing by a couple of inches and still be in a good range. 

Canoe, straight shaft

With the correct length of canoe paddle your upper hand will be in front of your face and the blade will be buried just to the shaft as you take your stroke.

To size your paddle, sit on a chair with back straight (your legs don't factor in how tall you sit in the boat).  Place the paddle with grip down on the chair between your legs.  The throat of the paddle (where the shaft meets the blade) should come to about the middle of your forehead, or approximately 6" above your chin. An inch or two longer or shorter doesn't matter too much.

Recreational canoe, bent shaft

Sitting on a chair as above, the throat of the paddle should come to about eye level.  Note: your overall bent shaft paddle length is often about 4 inches shorter than your straight shaft length. For example if you use a 54" straight shaft paddle, you'd use a bent shaft paddle around 50".

Competition canoe, bent shaft

Sitting on a chair again, the throat of the paddle (the joint between shaft and blade for ZRE carbon paddles) should come to the tip of your nose for C2 paddling.  A C1 paddle should be about an inch-and-a-half longer.

Whitewater

There are a lot of differing opinions on how to correctly size whitewater canoe paddles.  Factors include whether you paddle with your lower hand above the gunwale, how high/low you sit in the boat, whether you are on a seat or saddle, whether you are paddling solo or tandem...  

Generally speaking, start with your straight shaft paddle length and figure these factors in to determine whether you need to go longer.  Most whitewater canoe paddles are 56" to 58" long.

Dragonboat 

Choosing the correct size of paddle for dragonboat involves looking at a lot of factors such as body and arm length, boat type, stroke rate and your position in the boat (among other factors).  For a comprehensive guide check out Grey Owl's sizing chart here.

SUP

An easy way to find the correct length of SUP paddle is to flip it upside down so the grip rests on the floor.  The throat of the paddle should be at approximately eye level.  Another way of measuring is to pick a paddle about 10 inches taller than you, overall.  (If you are surfing or doing whitewater in your SUP, pick a paddle 8 inches taller than you.)

Kayak   

For a quick way to roughly measure a kayak paddle, grip the paddle so your elbows are just inside 90 degrees.  If you imagine a line from the centre of the shaft to the throat, your hands should be two thirds of the way down the line.  Remember, this is a very rough measure!

It is essential you take the width of your boat into consideration, and whether you are buying a low-angle or high-angle paddle.  For that reason, it's important to check a chart.  Werner Paddles has a useful fitting process here.

Happy paddling everyone!