In this paper, the effects of three visual factors: scene complexity, stereovision and motion parallax on correct perception of a virtual object's size were analyzed in an immersive virtual environment. We designed a controlled experiments set to incorporate visual conditions that reflected all twelve different configuration combinations of the three visual factors. Under each visual condition, subject performed the task of making judgments of the sizes of a virtual object displayed at five different distances from him/her. A total number of eighteen subjects participated in our study. The subjects' judgments and the corresponding actual sizes of the virtual object were recorded. Based on the colleted data, two quantitative measures of subjects' performance were derived and analyzed. The results of our experiments were consistent across the majority of the subject population and suggested that scene complexity and stereovision could have significant impact on the performance of a user of virtual environments to make correct judgments on a virtual object's size. On the contrary, motion parallax, either produced by the virtual environment or by the observer, might not be a significant factor in determining that performance.