I have a 20L tank for a young BP and I like it a lot. It is big enough to have everything you need inside, and not be too cramped.

I have a standard low wattage black light bulb for night viewing, and I love it.

If you have problems with humidity, which I anticipate with a screen top and a lamp, then I would recommend covering the screen with aluminum foil. Secure it with duct tape, and leave a hole about the size of your lamp for airflow/lighting/heat. This will raise ambient heat and also trap humidity.

That is the best advice I have gotten as far as husbandry goes.