How slim are they supposed to be? I know they're supposed to be thin, but how do you know when they're too thin or if they're overweight?

And I'll have some type of locks on all of my cages. My sister had a king snake whose name SHOULD have been Houdini!! She spent more time escaped than in a cage! (Not exaggerating, either). I don't want to deal with that, EVER. Especially with a retic - dwarf or not.