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 lighting fixtures you want to control. As a general rule, you'll want to choose a dimmer switch that can handle at least 10% more wattage than the total wattage of the lights you're connecting to it.
For example, if you have three 60-watt light bulbs, the total wattage is 180 watts. In this case, you'd want to use a dimmer switch rated for at least 200 watts. This ensures the dimmer can handle the electrical load without overheating or failing prematurely. It's also important to match the dimmer switch to the type of lighting (incandescent, LED, etc.) for proper compatibility and performance.