As mentioned above, a GP has to be longer than the usual sea kayak length to reach the water in a wider canoe.
Length should be planned with consideration of the boat width, paddler position (sitting on seat, kneeling or sitting on bottom pad-pack canoe) and last the height of paddler and are they long-waisted or short-waisted.
This sounds complex, but it's actually simple: have paddler sit in canoe in the position they will be paddling it. Increase the paddle length until the blades are fully in the water while you are paddling-that's your length for that canoe. A much wider or narrower canoe or a much taller or shorter paddler may require a different length.
If, after use, you decide you want a shorter or narrower paddle, use a saw or block plane to alter the one you have. For greater length make another. When you learn to make your own GP a longer or wider paddle can be made with 4 hours of time and $20-$40 of wood-not the $200-500 one has to spend for a different length Euro blade.
Beginners can find complete instructions for making a GP in the excellent pamphlet Brian Nystrom sells: "Greenland Paddles Step by Step"-google his name to find his website.