The traditional way to get the proper length of paddle was to line it up from your nose to toes.
That doesn’t work. The problem with this technique is that you are only interested in the length of the paddle between the grip and the throat of the shaft.The blade length is not part of the formula. It also does not take into account the kind of canoe you’re using, the height of your seat, where you’re seated (bow or stern), the length of your torso and arms, and the style of paddling you are used to.
The best way to measure the proper paddle-length is to sit in your canoe and measure the distance from your nose to the water. You can also sit in the canoe while it's in the backyard and just position the paddle beside you, upright and upside down. Then measure the distance from the lawn to your nose.
However, both approaches don’t help when you’re standing in the aisle of an outdoor store trying to pick out the right paddle.
So, try this instead: Grip the paddle with one hand on the grip and the other at the throat (where the shaft meets the blade). Then place it over your head. If your arms are perpendicular to your elbows, then it fits. If your arms are bent outwards, then the paddle is too long. If your arms are bent inwards, then the paddle is too short.
Keep in mind that this is not a definite solution. For example, I prefer a longer shaft when solo paddling than when tandem. And in a difficult set of rapids, I’d rather use a much shorter shaft than usual.
But it's a good start!