Tank size and humidity

Question
I have a Water dragon in a 20 gallon tank and looking to make a custom tank for him.  I would like to know how big of a tank a full grown water dragon should have L x W x H.
I would also like to know how can I keep his humidity level up in the tank.  I use wood bedding and soak it once a day plus spray the tank walls few times a day it keeps the humidity level up, but I have seen few people that have water dragons and they use green plastic turf for bedding and their tanks dont look moist like mine.  Not sure how their thanks humidity level is, but how important is the humidity and keeping it high at all times for a water dragon..   I know they come from a high humidity climates.  But having high humidity 24/7 in a tank is impossible to do.  If there is a different way to keep humidity level up?

Answer
The bigger the better as far as the cage goes.  I have kept water dragons in 5'x6'x2' cages with irrigation systems but I found that to be a giant pain.  I have also just given them free reign of the house and had success with that though it can be a little messy.  A simple answer for the humidity is just mist the cage maybe once a day or so make sure you have a large container of water.  They do need slightly high humidity but too high of humidity and they can develop blister disease.  If you are still worried about humidity once you get the dragon into a larger habitat you can try an ultrasonic fogger they seem to keep the humidity fairly high but like I said keep in mind you don't want the cage too wet.