If you get a Clamp Lamp get the biggest one, the bulbs are large and barely fit in the smaller ones, they need a good circulation or air around them to stop them overheating.
Personally I use the T-Rex Dome Reflector which is much better for the purpose, you can get something similar for chickens which is cheaper.

The wattage of the bulb isn't directly related to the size of the animal BTW, it's more to do with the size of the table, the bigger the table the more powerful the bulb to heat and light it adequately.