If you want to go with a belt, you can try the Gates calculator.
http://ww2.gates.com/first/resources.html
I like Gates belts for the drive train and I've found belts to also work well for low force applications like an intake roller.
In 2014 we used chain to transfer motor torque on our shooting arm. Chain is more tolerant of lateral mis-alignment and the tension variation than belts.
Dave