18 x 18 x 24 for an adult crestie or gargoyle would work.

Hatchling should be started it in a small tub or in a 10 gallons tanks to prevent stress and inability to find their food.

I have no experience with leachies but this is a great source, http://leapinleachies.com/articles.htm?article=7 , http://leapinleachies.com/articles.htm?article=5