You can do it one of two ways -
When i first got a 5 week old 6 inch long baby, he went right into a 40 breeder tank. No problems - just keep tank decor minimal so they can find prey easier. You can also use a feeding tub . You get better temp gradiants in a larger tank.
I wouldnt go smaller than a 20 long to start off with for a baby, if you don't want to get the large tank first. For an adult Beardie , they can be okay in a 40 breeder, but a 60 would be even better. Or, if you're industrious, make your own habitat
4'Lx2'W
Hope this helps!