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A recent algorithm of Modiano selects packet sizes in a selective repeat ARQ protocol based on the acknowledgement history of the most recently transmitted packets. In this paper we modify this algorithm so that the choice of packet size is restricted to one of two pre-specified values. We provide a strategy for switching between these packet sizes and show that is optimal in the sense of maximizing the one step efficiency. The throughput efficiency of the proposed adaptive scheme is analyzed for a constant bit-error-rate channel and for two state Gilbert-Elliot channel. The results show that the throughput efficiencies of this scheme under high and moderate bit-error-rates are slightly less than that of Modiano's algorithm. However the scheme is attractive because of its simplicity.