The answer to 'what size switch do I need' is that the size of the electrical switch you need depends on the current and voltage requirements of the circuit you are powering. In general, you want to choose a switch that is rated for a higher current and voltage than the circuit will be carrying.
For example, if you are wiring a 15 amp, 120 volt circuit, you would want to use a switch that is rated for at least 20 amps and 125 volts. This provides a safety margin and ensures the switch can handle the electrical load without overheating or failing. Switches come in a variety of sizes, from small toggle switches to larger rocker or paddle switches, so make sure to select one that is appropriately sized for your circuit.