The answer to 'what size switch do I need' is that the size of the switch you need will depend on the electrical load it needs to handle. The most important factors are the amperage (current) and voltage rating of the switch. For most household circuits, a standard 15-amp or 20-amp switch is typically sufficient. These sizes can handle the electrical load for common lighting, outlet, and small appliance circuits.
When selecting a switch, make sure to choose one that is rated for the voltage of your home's electrical system, which is typically 120 volts in North America. The switch should also be rated for the same or higher amperage as the circuit breaker or fuse that protects the circuit. This ensures the switch can safely interrupt the flow of electricity if needed.