I think the AJ-260-2 is the biggest rocket engine ever built, although it never flew. It was a 260 inch solid rocket motor built to determine if they could replace the Saturn V first stage with a single solid booster. The motors tested were half the height of the real ones but still produced over 17000kN of thrust the full size motor would produce twice as much thust. Apparently when they test fired it you could see the flame 50 miles away. The link below tells more, the astronautix website has everything you need to know about space flight.