I have my customers size their paddles via shaft, since as was
previously mentioned the blade length will vary.

My quick and dirty method of sizing is to take the paddle and have
them hold it over their head. Their arms will be out, with their
elbows at 90 degree angles. Basically it looks like they are trying to
bench press the paddle.

I then look at the distance from their hands to the blade. For racing,
1-2" seems a good fit. For touring 3-4" allows a lower stroke. Over 6"
really feels really long. My all around paddles are 2" on me, though I
use a Length Lock and adjust for load and boat width.

This assumes a higher paddle angle, or race stroke. The lower the
paddle stroke, the longer the paddle.

I am 6'5", and when paddling a Seda Glider at 20" wide, I opt for
something in the 216-218 range. Wider boats will use a slightly longer
paddle, and in something 24" wide, I may use a 220-222cm