Also, apparently too long a paddle can mess up your shoulder. The following recommendation appears online. How does this sound?
What length paddle do I need?
Paddle length is important for technique, posture, and efficiency. If too long or short, it could cause joint and back injuries, and also not be efficient over long periods. The rule of thumb is to have a paddle "one shaka" (6-8 inches) over your height. Longer is not better and could even cause shoulder joint injury.
Your height plus 7 inches is the magic number.
From:
http://www.c4waterman.com/stand-up-p...do-i-need.html