Plastic injection molding process is an advanced but much complex plastic part molding with multi-factors coupling interactive effects. The size of the part has a significant effect on the injection molding process. In the current work, a kind of common plastic part, common cup, is taken into study. Based on the 3D FE model, the effect laws of size of the common cup on the fill time, pressure at V/P switchover, clamp force needed and maximum deflection considering all effects in the injection molding process have been investigated. The research results show the following: (1) With increasing the initial diameter of the cup, fill time, pressure at V/P switchover and clamp force increase obviously, while overpacking happens for the maximum deflection considering all effects. (2) With increasing the initial height of the common cup, fill time, pressure at V/P switchover and clamp force increase less obvious. Meanwhile, overpacking doesn’t happen. (3) The effect of height to diameter ratio H0/D0 by changing D0 on injection molding process is different from that by changing H0.