red-eyed

if im building a tank for a beardie or water dragon how many gallons /
what approx dimensions shud i have?? books that i have read, say for
adult beardie to have a four foot long tank but loadsa people at pet
shops say that as beardies dont do a lot they dont really need that
big of a tank and that a water dragon would benifiet better from that
size tank as they do loadsa jumping / running? going to biuld i#one
soon so need as much advice as poss. cheers