Thanks for the reminder to always read the rules. There is a minimum width, but I saw no miniumum height (aside from that implied by the requirement to have 4x4 skids on the bottom of the crate). There is definitely a maximum height. If there is a minimum height, could you please quote the applicable rule? I could have missed something.
http://usfirst.org/roboticsprograms/...nt.aspx?id=452