If I had to say a size I would say maybe 8-10" overall length. Small veileds can have problems as they simply just overheat quickly. A red flag that he is to hot would be him basking with his mouth open.
Perhaps trying a different type of bulb if you can not get the heat up any more. Halogen and flood lights (ones with the metal on the stem) tend to generate a little more heat without having to go up to a crazy wattage bulb. You can also move the branches (or vines) around to get in a more favorable spot or light.