The answer to 'what size recessed lighting do I need' is that the size of recessed lighting you need depends on the size of the room and the desired level of illumination. As a general rule, you'll want to use 4-inch recessed lights for smaller rooms and 6-inch recessed lights for larger rooms. The number of lights needed will depend on the square footage of the room, with a good guideline being one light per 4-6 square feet of floor space.
When choosing recessed lighting, it's important to consider the wattage and lumens. Higher wattage bulbs will provide more light output, but you'll want to balance that with energy efficiency. Most standard recessed lights use 6-inch cans and can accommodate LED bulbs in the 12-18 watt range, which will provide ample illumination for most residential spaces.