Personally I don't think the 12" cubes are enough space for a larger adult. If you are looking to get a pair, a 12 x 12 x 18 is even cramped. I would suggest a minimum of a 18 x 18 x 18 as a suitable size for an adult pair.
Also keep in mind that if you get juvis you will have to seperate the males from the females as they grow to be sure that they don't breed too early.
I would suggest going with a tub set up at first, they are inexpensive and easy to upgrade as the geckos grow. It would also alleviate your concerns about them being too small for their enclosure.