Matching a drive shaft to power and speed is a standard race car problem and I've seen formulas for calculating diameter and wall thickness at various lengths and speeds. I know of at least one online calculator aimed at the race car crowd.
Drive Shaft CalculatorWhile I can imagine some guy calling a drive shaft unlimited I suspect what he meant was a drive shaft with huge margins beyond the probable maximum requirements. Maybe a carbon fiber shaft?
I'd probably make the guy admit his drive shaft was not unlimited or I'd choose to go somewhere else. I don't like hyperbole from vendors.
