Check out my video on how to choose SUP paddle lengths.
This a basic starting point length. Many over time may choose a longer or shorter length per their body type, type of board (thickness), and type of paddling.
Some surfers like a shorter paddle for a quick cadence in paddling out or for tube riding. I
like my normal length paddle in surfing for additional bracing support down a wave.
Starting with an adjustable paddle gives you a chance over time to figure out what length is best. Then if you buy a one piece carbon to your fav length, keep the adjustable as a backup or for friends who need a paddle.
Paddle length is done by inches (in the US) from the blade length to the top.