1. Arm length and torso length.

2. Boat width.

3. Paddling style (low-angle vs. high-angle stroke).

A good general rule is to grip the paddle where you normally would for paddling (where the grips are on the paddle if it has grips) and then hold the paddle in the air and centre the shaft on your head. Your elbows should be at right angles.

From there you can go up or down a size based on the type of kayak you're in: longer for wider kayaks and shorter for narrower kayaks.

You can also go longer or shorter based on the paddling style you're going to use: longer for a low-angle, relaxed stroke; shorter for an aggressive high-angle stroke and a fast cadence.

One of the best guides to paddle length that I've found is the online Werner Paddles Fit Guide. It sized me at exactly the length that I prefer. You can play with the variables and see how lengths vary based on the type of boat and your paddling style.

http://wernerpaddles.com/fit_guide/index.php