Tom,
It will depend on where you are going to attach it; wrist or boat. The store bought one I have is 36". This worked fine on my 27" wide boat attached to my wrist when using it for photography. I took it off after five miniutes paddling the new Night Heron because it spent more time in the water than "on deck". Start long and cut it down as needed. I think the perfect length will be different for everyone. You should also look into a "paddle park" which my freind uses to fish. They are a couple of plastic hook devices you can attach to your hull if you are not familiar with them.
Timothy Mulherin