There are two different types of Fire Fans. There are Russian and American (Kaos) style fans. Kaos style fans are more appropriate for a lot of geometric tech and tend to have several small rings, while Russian fans have one large ring and are better for a style closer to Poi.
Thinner wire leads to a delicate prop that can break after being dropped repeatedly, they will also heat up quickly when used. Thicker steel rod provides a more durable prop but is more difficult to change planes with, both when spinning and rotating the fans.
Because fans are heavily weighted towards one end, the size of the ring (if present) has little bearing on the maximum speed at which the fans can be spun.
Rings may be left untreated, have a fire-resistant paint coating, or use leather to modify grip and comfort. Larger rings located at the end of the fan can be distorted when dropped, and are often encompassed by an O- or D- shaped ring for protection. Sometimes people will also use gloves if they find their rings to be uncomfortable, though this has become less common.
Without a welder it is nearly impossible to make a reliable pair of fans. Fans can be purchased online for about $60-$200, with prices generally around $100.