I've participated in (and now have been a ref for) a robotics competition, First Lego League, a couple of times. The robots can have a height of up to 30.5cm (12"), and that's usually quickly checked before each round.

Since there's pretty much always some A4 paper lying around near you, probably a copy of the rules, in most situations we just hold up a sheet (rolled up if you're feeling fancy) next to the robot. Much quicker than finding the ruler that's probably in use (or misplaced) somewhere else :D