This is where one of those cheap orange fish grippers I favor would come in handy.

They have a bungee cord for your wrist. When you bring that monster along side and its temporarily exhausted, clamp on and take your picture with your free hand. This is where a small digital camera comes in handy. CVS sells a waterproof " throwaway" digital sports camera for cheap. Strictly point and shoot, easy with one hand. I take one and keep in shirt pocket for quick shots. If it starts thrashing, you can easily un-clamp with one hand.

You could also lay your paddle next to fish in water ( assuming your spare is handy ;)), and use it for as a way to determine scale/length and keep the tale factual ;D

The fish grip floats, so that's not an issue. For under $20, I don't think there's a better fish aid out there.