You got the cart before the horse. First, you compute the combined loads, using diversity factors allowed by the NEC. Second, you choose a panel of a size adequate for the loads just computed. Third, you choose a feeder of ample size. Fourth, you choose a circuit breaker sized to protect the feeder and panel.
Or, take the shortcut: size the breaker for the size of the new panel (usually 100 or 60 Amps)and feed with 100 or 60 Amp wires.