Measure the beam of your kayak, (the center of the cockpit at its widest point). If the width of your kayak is 25 inches or less, and you have an average torso length, the following rule of thumb, (Step 2), should get you close to your proper paddle size.
Hold the paddle upright and stand next to it while extending your hand to the tip of the paddle. If you can curl your fingers over the tip of the paddle, the paddle is the right length for you.
Quip: A bad day kayaking is better than a good day at work!