I used a sort of hybrid arm. I selected an arm in black epoxy from the same people that made the motor, so it fit the motor stud perfectly, then attached the wiper blade end from a conventional auto-type wiper. That allowed me to select and use a regular auto-style wiper blade from a wide range of sizes.
I think I ended up with a 19" blade, but every car will likely require a different size, depending on how high the motor is mounted in the body. To get the right size arm, I dusted the windshield, then ran a few wipes with a very small wiper blade. Once I got the pattern, it was easy to measure for the optimum blade length.