These are also called leachianus geckos (or "leachies"). Mine is in an 18"x18"18" enclosure which is smaller than what you're considering. They are large, but they don't move around much. I find that she uses the whole enclosure, meaning that she's in different parts of it at different times but she's not roving around. I have a large round piece of cork bark that she likes to spend time in as well. If you're getting a juvenile, it will be best to keep it in a much smaller enclosure to start with. When I got mine, nearly 10 years ago (!) it was pretty small and I kept it in a 6qt shoebox sized enclosure until it got to be about 35 grams.
For a juvenile: pros of a smaller enclosure is that they can feel safer and it's easier for them to find their food; cons is that you have to have the starter enclosure and the adult enclosure.
For an adult: pros of a smaller enclosure is that it takes up less room and is less expensive. Cons is that if it's too small, the gecko doesn't have much room to move around.
When I got my first crested gecko 16 years ago (he's still with me!) I didn't know about the need to put a juvenile in a smaller enclosure and I put him, as a small1 month old, directly into a 12x12x18. He did fine.