According to ICAO Doc 9157 Aerodrome Design Manual, Part-1, Runways (2nd ed):
The aeroplane reference field length is defined as the minimum field length required for take-off at maximum certificated take-off mass, sea level, standard atmospheric conditions, still air and zero runway slope, as shown in the appropriate aeroplane flight manual prescribed by the certificating authority or equivalent data from the aeroplane manufacturer.
Similar definition exists in ICAO Annex 14, Chapter I - General:
Aeroplane reference field length.
The minimum field length required for take-off at maximum certificated take-off mass, sea level, standard atmospheric conditions, still air and zero runway slope, as shown in the appropriate aeroplane flight manual prescribed by the certificating authority or equivalent data from the aeroplane manufacturer. Field length means balanced field length for aeroplanes, if applicable, or take-off distance in other cases.
Regarding Boeing 737 narrow runway operations some descriptive information can be found in Boeing presentation entitled "Guidelines for Narrow Runway Operations" by Rob Root, Flight Operations Engineer, Flight Technical Services, Boeing Commercial Airplanes. I guess you can all find the document elsewhere on the net.