Honestly, I have never had a dragon afraid at how big his tank was. I don't know how the idea of big tanks are scary for babies got started, but IME its not true.
If you are going to be feeding insects in the tank, then it would be better to part some of it off, but if not there is no reason why your dragon can't be in a 3000 gallon tank assuming you can keep the temps right and provide uvb through the tank.
-Brandon