Definitely go with the largest enclosure you can get. If all you have for now is a 55 gallon, then thats what you use, I suppose. You should know that it restricts the movement of your adult dragon, which can cause issues. It also makes getting a good temperature gradient more difficult. If you have the 55 gallon already, then make use of the extra height by providing lots of branches and vertical furniture.
Please do not mount your UV lights to the back or sides of your enclosures, especially if you have logs or other furniture that allow your dragon to climb. UV light is damaging to sensitive eye tissue, so they need to be mounted to the ceiling, directly above. That way the natural adaptations of the dragon to protect against UV damage is in place for them. If you get a T5 high output light from either Zoomed (10%) or Arcadia (12%), they are the best fluorescent tubes available. Make sure you get a fixture with a reflector in it. Otherwise you are basically wasting about 1/2 to 2/3 of the output of the bulb. You can also take a look at the ExoTerra metal halide light. They are quite a bit more expensive, but they are also better than fluorescent tubes as they produce a much broader spectrum of light and have a lot of UVA as well. They make a lower wattage version now that has an absolutely fantastic spectrum. For an enclosure that size I wouldnt recommend any higher than the 70 watt.
As far as what wattage for the basking bulb, it will really depend on how far away the basking area is. You shouldnt need any higher than a 75 watt flood light if its within 12 inches or so.