If you get a male, you're going to need a 24X24X48 at around 6 months anyways.
Since you shouldn't purchase a chameleon really younger than 3 months, especially if its your first time.. No matter what size baby cage you get, it will only really last 3-4 months.
If you go to cages.net and click on "Todays special value" they have an 18X23X11 that I like for $35.
But the best advice I can give you is to save the $35+Shipping, and considerable expense that feeding a baby cham can be.. and apply that money towards a 6-7 month old veiled. That way you only have to buy one cage, and you will have an animal that is well established.