Well, it doesn't have to go the full length of the tank since the UVB should be placed alongside a white basking bulb so your beardie can get the benefits of both bulbs while in the basking area. But, would suggest at least a 24" (18 watt) or a 36" (30 watt). If you order before midnight tonight, you can get 10% off your order by putting HOLIDAY in the check out section under coupon code.