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 room and the intended purpose of the lighting. As a general guideline, you'll want a light fixture that is roughly 1/2 to 2/3 the width of the room. For example, in a 10x10 foot room, you'd typically use a light fixture that is 5-7 feet wide.
The wattage and lumens of the light bulbs you use also play a role in determining the right size fixture. Brighter rooms or task lighting may require higher wattage bulbs, while ambient lighting can use lower wattage. It's best to measure the room, consider the lighting needs, and then select a fixture size that provides the right amount of illumination for the space.