In an enclosure that size I would replace the T8 fixture with a 24'' T5 fixture. A good T5 is a much better choice than a T8 except in very small enclosures. As far as bulb wattage goes, that really varies between setups. When you have a fixed top like that it's helpful to use a halogen bulb so it can be dimmed easily. You still want to get it just about right though which can take some trial and error. I'd start with maybe a 65 or 70w halogen flood light and move up from there as needed. Shoot for a basking surface temperature between about 100-110 and a cool side surface temp between about 75-85. Adjust the wattage until you get close to that and then you can fine tune it using a lamp dimmer or dimming thermostat.