This chapter presents the studies and analysis on the approaches, the concepts, and the ideas on data packet size optimization for data packets transmission in underwater wireless sensor network (UWSN) and terrestrial wireless sensor network (TWSN) communications. These studies are based on the related prior works accomplished by the UWSN and TWSN research communities. It should be mentioned here that the bulk of the studies and analysis would be on the data packet size optimization techniques or approaches rather than on the communication channel modeling, but the channel model is deemed essential to support the optimization approaches. The various optimization solutions proposed in the prior arts are dealt with in depth to explore their feasibilities to accommodate the data packet size optimization algorithm proposed by the various researchers. This chapter starts off with the studies and analysis on prior arts found in UWSN and then moves on to the similar works found elsewhere in the TWSN communications counterparts. A comparison on some important issues related to data packet size optimization approaches used in UWSN and TWSN communications are summarized in a table at the end of this chapter. The findings in this chapter may be helpful to readers who are interested in the R&D of data packet size optimization techniques with the intention to formulate new data packet size optimization framework or algorithms. © 2012, IGI Global.