The answer to 'what size light fixture do I need' is that the size of the light fixture you need depends on the size of the space you want to light. As a general rule, you'll want a light fixture that provides 30-50 watts of light per square foot of the room. So for a 100 square foot room, you'd want a light fixture that provides 3,000-5,000 watts of light.
The specific size and type of light fixture you need will also depend on factors like the ceiling height, the layout of the room, and the style you're going for. It's a good idea to measure the space and consider the overall lighting needs before selecting a fixture. If you're unsure, it's best to consult with a professional electrician who can assess your specific needs and recommend the right size and type of light fixture.