Bearded Dragon Tank..Size?????

[GP212]

Hello. I am new to this stuff. We (me and my kids) are planning on purchasing a Bearded dragon as soon as they get some in at the pet store. I have been doing my research on habitat but I am still unsure on the size of tank that I should get. Is 36x18x18 good(50 gallon). The have one that size at the pet store with sliding doors at the front. Or should I go for the 65 gal which actually is cheaper but has no doors on the front(I thought that they were a nice option). And I just saw a 100 gallon in the paper which won't hold water so it is cheap. 72x20x20 .Too big?? What if we were to end up with two BDs some day? How big would the tank have to be?
Just wnat to do the right thing the first time.

Thanks!!
GP212