If you can convince your parents, the 18x18x24 is the best. I have a 12x12x18 for my Azy and it dosn't seem to be enough space for her.
Either way what you'll need is up space and plenty of it. Large pieces of wood (large enough for him to sit on comfortably) that can be arranged in a horizontal-vertical (more horizontal then vertical but still going slightly up) that he can climb up on. You'll also want hiding places. A good sized broad leaf plant such as a Pathos or similar is good because the vines can be arranged around the wood to make places he can hide in. Azy had a favorite place in her half log I bought from Petsmart (the large one) but I had to throw it away last night because it was covered in mold on the inside. I'm still trying to figure out how that happened but I'd assume it was the lack of breathing room (in the 12x12x18) so the moisture in the substraight soaked right into the wood and created mold. If you can get the larger tank you might be able to avoid the issue and these half logs are really nice little hidy holes for them. And the water dish should be just a bit larger then he is. If you can take a bit of yarn or something and measure how long he is and how wide he is then pic a dish just slightly (no more then half an inch) bigger then he is long (the width should be right if its the right length).
Hope that helps. And I hope you can get the bigger tank. ^w^