the ideal hide-height is 0" (until they dig under it). I let them do it, because I don't know exactly what they want their hides to be like, so I just let them dig into the dirt, and usually the height of the hiding areas they construct are surprisingly low. My two mali's usually make it low enough that they have to squish themselves inside. The young mali squishes herself into areas you'd think she would never fit. Basically anything as tall as her head, she likes to hide under.
The tortoises are even funnier, as they just bury themselves in dirt. They have plenty of hiding areas, plenty of dirt to build burrows, but they bury themselves around early afternoon and surface the next morning. It's somewhat bizarre.
But basically, almost all reptiles (aside from arboreal) like to hide in spots where they can feel the floor on their stomach and the ceiling on their back and that same time, and they can be touching at least three sides of the chosen hiding spot.
In general I just try to remember that it's their hiding spot, not mine. Who am I to choose what specifications they like to hide in?