Propeller Size and Motor rpm for RC Plane?

i am new to Rc's i need help with the propulsion system? for example:- aircrafts max payload is 6kg and if we are using a single motor engine then 1) how to decide the propeller diameter 2) how to decide what should be the rpm for the motor and motor's size? need the exact equations, it will be really helpful for further calculations if i want to change the max payload, thank you