Step 3: Rip Sticks
Ripping is one of those jobs that's a whole lot easier with power tools. I've done it with a hand saw, and it's a day-eater.
1.25" diameter is about right for a round softwood stand up paddle shaft. 1.5" is too thick. 1" wouldn't be too thin for the hand, but is too flexible for a shaft this long.
Measure some paddles that look and feel good to you to see how thick you want yours to be.
Personal preference will vary greatly. A rectangular shaft can be thinner than a round one and have the same strength.