I was looking at a Exo Terra® Natural Terrarium and it said to pick my size well it has like 18x 18x 12x, 24x 18x 12x,and 36x 18x 12x.
I don't know what size is good for a bearded dragon.
Beardies need ground space more than anything so you should pick the longest and widest one. But this still depends on the age of your beardie. Babies should have a 10-20 gallon tank so it will be easy for them to hunt for crickets and it is much easier to be accustomed to his/her new surroundings.