Size all depends on how many leos you are putting in the sweater box. For my one adult leo (spoiled as she is), she is in a 90qt container. You can obviously go smaller, but when looking last night at Walmart for other sizes, the smallest size I would recommend for an adult is a 30qt, because they are the largest floor space allowed until they start getting really deep. As far as holes, make sure you drill some on each side of the tank, as well as all along the top. I personally did about 30 holes on the front, 15 on each short side and about 30 on the back corners in 3 rows of 5 on each side (kind of different than my 3 rows of 10 across the entire front). I also drilled probably 40 holes all along the top of the lid to make sure the air circulation is good.
Humidity could be an issue for several reasons...one, what substrate do you use? Also, the air circulation without holes in the top can cause major problems. Another thing to think about is how much water do you have, and is it sitting on the hot side of the tank? If so, the water will evaporate faster and cause humidity build up.
Hope this answers some questions, and post up some pics when you get it done!
Kelly
