Are 40 gallon terrariums recommended for adults?

Here is what my gecko looks like, I don’t know about you, but he looks too big to be an adult. And his tank is a 20 gallon, recommended for juveniles. I heard on a video, saying that 40 gallon terrariums are for adults, and I would like to know if this is true or not. (Note: the photo of his tank is an old photo; I stopped using sand due to my dad’s concerns (as well as other people’s concerns), and not having reptile soil to mix it with, so it’s still a WIP.)