Modelling traffic generated by Internet based multiplayer computer games has attracted a great deal of attention in the past few years. In part this has been driven by a desire to properly simulate the network impact of highly interactive online game genres such as the first person shooter (FPS). Past work has dealt with packet size and packet interarrival times without consideration of packet size autocorrelations. Packet size auto-correlation is an important element in the creation of plausible traffic generators for network simulators such as ns-2 and omnet++. In this paper we present an analysis of the auto-correlation of packet size for Half-Life, Half-Life Counterstrike, Half-Life 2, Half-Life 2 Counterstrike, Quake III Arena and Wolfenstein Enemy Territory. We show that packet sizes appear to be autocorrelated.