Personally I'd go for 18" with a shelf (not a "sky-hide)- I've had a couple of HDPE enclosures in the past that were only 12" & I hated the lack of height- it's miserable to reach into for cleaning, & there's not much head-room for things the snake can climb on. Consider the type & depth of substrate you plan to use, & also the type(s) of heat- many things use up some of that space- 12" is really minimal, & 24" is too much (harder to heat).