To start off a little background information. According to the IDBF paddle specification 202a, a dragon boat paddle must be at minimum 105cm (~41 inches) and at maximum 130cm (~51 inches) in length. This specification refers to the overall length of the paddle from the top of the handle to the bottom of the blade. All paddle manufacturers keep the blade at a constant length and adjust the overall length of the paddle by lengthening the shaft.
What does a longer paddle do?
It is usually a general consensus that a longer paddle is better, but what exactly is the effect of having a longer paddle? It's safe to assume that your arm length will remain constant, regardless of what paddle length you use. Also, it's safe to assume that your bottom hand height is constant at the catch since the blade length is constant and your hand should be placed just above the blade (you are doing that right?). So when you go with a longer paddle, what's moving is your top arm.
|Longer paddle length vs. top arm angle.|
|Longer paddle length at limited shoulder mobility|
|Paddling with a short paddle|
Your optimal paddle length will depend on your top arm's overhead mobility, your torso's rotation and oblique/lats mobility. This is the "experience" level stated by paddle manufacturers. So now let's address their other measurement: your height. Well your overall height is actually not a good metric for paddle length, however what does have an effect is the length of your torso and your arms. So if you want to good way to ballpark an appropriate paddle length, sit down on the bench and stand your paddle next to you (blade on the bench, and top arm over the handle). If you can barely reach the top of your paddle, then the paddle is probably too long for you. If you can reach the top and your elbow is extremely bent, then the paddle is probably too short for you. Ideally, you should have a slight bend to your elbow while being able to comfortably grab the handle of the paddle. From there, check your catch angle and adjust accordingly with a longer or shorter paddle.