Hi Paperboy, welcome to the new world

Please, follow what Colas, JeanG and Pete said .
My input is: paddle length will be length needed to get the top of shaft at head length and paddle in water as Colas did show on his picture up there.
The length will be up to the body position
you can afford. It will take times to get it . So cut shorter and shorter shaft of your paddle progressively until you get an optimum paddling position as Colas did. Like JeanG said " practice"

Personally, during my last SUPing trip with 6'5" 28" , took me 2 weeks of daily sessions to be in physical shape and able to stand the perfect paddling position. I use to do stretching along improvement lower body strength to perform this training.
Once you get the proper paddling position and your own paddle length , this is a new world of surfing opened to you.
So follow and practice what the shortSUP chaps said