The problem of determining page size in a page on demand system is discussed in detail in this paper. After having introduced the problem, the effect page size may have on various system performance measures is reviewed based on measurements of program behaviour and on simple models of system behaviour.This discussion is followed by a detailled study of the effect of the choice of page size on the efficient utilization of primary memory space. The wasted space-time integral (WSTI) of primary memory space is selected as a measure of this utilisation and a new model of program and system behaviour is used to compute the WSTI for different secondary memory devices (drum, ECS and LCS) and different system behaviour parameters such as the time spent in supervisor mode by the operating system to initiate a page transfer, the time spent in the CPU queue by a program which has recovered from a page fault before it receives the attention of the CPU, the global page fault rate (or arrival rate at the secondary storage devices used for paging) and other factors. The influence of each of these factors is discussed and analyzed, and the conditions under which one or another of these dominates the problem is identified.