Okay, tried to answer my own question with some Web research. The claim, see below, is that it's 15" x 10". I could swear I've gotten luggage through that was maybe about 12" high (laid on the conveyer) but maybe I'm imagining things. Or could it be that templates are different at different airports.


from http://urbantraveler.com/urban_travel.html
"People are always asking about that United Airlines template. You know, the square metal thing on the security belt that your carry-on luggage has to feed through in order to be legal carry-on size. The template says 14" x 9". This concerns people because most wheeled carry-on luggage are 14"x9" not including the carry handle and other little bumps that can add up and make the bag about 14.5"x9.5". So I measured a United Airlines template at LAX international and it was actually 15"x10". They can't fool me."