For a juvenile veiled chameleon, 16 x 16 x 30 is good, but the appropriate size really depends upon the type of chameleon you're getting.
If you click on Resources at the top of your screen, then on "Caresheets" on the left side, you'll find you can access caresheets for a variety of chams. Appropriate cage sizes for juvies, adult males, and adult females are given in the care sheets for each species.
Have fun preparing!