The worlds largest single engine fixed wing aircraft is the Air Tractor 802.

http://www.airtractor.com/models/802A/

Having stood next to this thing and set in the cockpit I can tell you that this thing in huge, especially when it is on floats and set up as a firefighter. Look at the size compared to the Aerostar twin sitting next to it. BTY the Aerostar is a 6 place airplane.



-Chris