We report on the construction and calibration of a 6 m2scintillation counter. The counter consisted of two 1.22×2.44×0.051 m pieces of clear Lucite which were optically joined along a common edge. Optically coupled on top of these were sixteen 0.61×0.61×0.019 m squares of NE 110 plastic scintillator. Light collection was achieved with eight photomultiplier tubes. This counter had an operational attenuation length of 4.8±0.9 m. Calibration of this counter with an anticoincidence counter to discriminate against multiple particle events is described. This anticoincidence counter permitted particles that passed through a small calibration area to be counted only if no other particles were passing through the remainder of the detector at the same time. Large area counters will often receive more than a single particle per event. Theoretical calculations for purely electromagnetic extensive air showers indicate that the 6 m2counter tested receives an average of 2.5 particles per event. The calibration data yield 1.79±0.08 particles per event. These results imply that if calibration on the response from a single particle of an extensive air shower is desired, it is necessary to use the anticoincidence counter method described for counters of more than a few square meters area.