The single largest surface should be measured as a horizontal length. The presence or absence of wing tips should not affect that measurement. If the horizontal stabilizer and wing are of the same length then the plane would not meet the event's Construction Parameters.
Both wing and stabilizer surfaces are measured to opposite ends, spanwise (port to starboard) in a projected view. When the aircraft is held level, both surfaces must pass through a go-no-go gauge set at maximum wing span. The surface that is measured to be longest in span will be considered the wing.