In gravity separation operation, the probability of a coal particle to appear in the product stream or in the refuse stream is a strong function of its size. Improper selection of feed size may lead to significant reduction in efficiency of the separation unit. Available literature conforms that the separation efficiency of an air dense medium fluidized bed separator, which is a gravity separation operation, decreases with feed coal size, but determination of appropriate feed size range, for efficient operation of this separator, still remains as a challenge. In this paper, the effect of feed size on the separation performance is investigated and an attempt has been made to quantify the optimum size range over which the separator can operate satisfactorily. The performance of the equipment is evaluated in two steps, namely, sharpness of separation affected by the washing unit and the efficiency of the washing operation. Predictive model equations are also proposed for describing the relation between sharpness of separation and feed size. Four Indian coal samples of different washability characteristics are considered for this investigation. Preliminary results show that the air dense medium fluidized bed separator performance is quite satisfactory for the feed coal within the 50-15 mm size range.