Chris,
The size of the metal parts of the connector are goverened by the current rating. They need to be large enough to take cable rated at 20 amps and the metal parts must also have a large enough cross sectional area to carry the current. The thickness of the insulation is determined by the voltage rating and the required physical strength. I do not think a terminal rated at 24 volts (If manufactured.) will be signifacantly smaller than one rated at 300 volts. I would have thought that the 24 volt circuit is only a control circuit. Not the circuit that powers the machine. More details of what the PCB that you are designing does and what it connects to would be a great help.
Les.