What is the righ size tank for one beard dragon?

It depends on how big you water dragon is...

A baby would need a 29 gal tank atleast...and adult needs a custom built tank because you really cannot buy one that's big enough...

My water dragon has a cage that's 3 feet wide, 4 feet tall and 3 feet deep....

They not only need lenght to run with, but they LOVE to climb, so they need height too...

A 55 gal tank is a good lenght but it gives them no where to climb...

I know many people who get shower setup's full shower that you can buy from Home Depot's their full grown dragon's need A LOT of space...

If you are thinking about getting a dragon and can't make it/buy it a large enough tank...then you may want to reconsider...

If you are willing to build a cage big enough but cannot do it for a year or so, then get a 29 gal tank and get a baby..

hope that helps