The answer to 'what size dimmer switch do I need' is that the size of the dimmer switch you need depends on the wattage of the lights you want to control. As a general rule, you'll want a dimmer switch that can handle at least 25% more wattage than the total wattage of the lights on the circuit.
For example, if you have 5 light fixtures that each use a 60-watt bulb, the total wattage would be 300 watts (5 x 60). In this case, you would want to use a dimmer switch rated for at least 375 watts (300 watts + 25%). This ensures the dimmer can handle the electrical load without overheating or failing prematurely.