In many packet switched networks, packet sizes are basically 128 or 256 bytes, probably due to their adaptation to interactive communications with short delay requirement. However, in order to deal with message communications also, characterized as bulk, unidirectional and delay tolerable, the networks may be required to handle a larger packet size suitable for the message communications characteristics. This paper calls packet switching systems, which handle various sizes of packets, heterogeneous packet switching systems. Focusing on packet transfer cost in heterogeneous packet switching systems, this paper discusses an optimum packet size for message communications. For an optimum packet size, an approximate and analytical solution is obtained when packet header length is ignored. Moreover, strict and numerical calculation results under a general condition are also shown. It is shown that the approximation gives a smaller size than the strict solution. Further, the effect of cost reduction caused by an optimum packet size for message communications is evaluated. As a result, the cost in heterogeneous packet switching is confirmed to be reduced to about half, especially when message communications traffic is dominant.