Existing psychophysical experiments show that size changes of objects could influence the human perception of objects properties (e.g., shape or weight) in the physical surroundings. On the other hand, Augmented Reality (AR) is an interactive toolkit that allows the integration of virtual objects and real-world environments. However, the absolute detection threshold of size changes on virtual objects is not clear in AR, limiting human perception and experience in AR. In this paper, we designed a pilot study and two psychophysical experiments to explore the humans' ability towards the range of the detection threshold of size changes on virtual objects in AR. Our experimental results show that the range of the detection threshold of size changes is 3.10%-5.18% (reference is a 10 * 10 * 10 cm 3 cube). We also calculated the value of the point of subjective equality (PSE = 4.00%), which means that users react 'same' with 50% probability upon the given stimulus.