The answer to 'what size ceiling fan do I need' is that the size of the ceiling fan you need depends on the size of the room it will be installed in. As a general rule, you'll want a fan with a blade span of 42-52 inches for rooms up to 225 square feet, and a 54-72 inch fan for rooms up to 400 square feet. Larger rooms may require multiple fans or a commercial-grade fan.
The most important factor is to match the fan size to the room size. Ceiling fans that are too small won't circulate air effectively, while fans that are too large can overwhelm a space. It's best to measure the room and consult a sizing guide or speak with a professional electrician to determine the right fan size for your needs.