Augmented reality (AR) has positioned itself as one of main media technologies, and its further proliferation depends on its usability, ergonomics and perceptual qualities with respect to the effective and correct convey of information. However, there exists no established guideline as how to visualize augmented information properly for various types of AR displays. We investigate for one important perceptual quality in AR, correct size perception of the augmented object, for three representative AR displays. The AR displays considered are: (1) small hand-held mobile device, (2) closed video see-through HMD, and (3) optical see-through HMD. The augmented object, a nominally sized box, is visualized in three different styles: (1) as a texture mapped simple polygonal model, (2) as a bump mapped polygonal model with shadow, and (3) as a detailed scanned mesh model. The size perception was assessed in two viewing angles: (1) sitting down and looking straight and (2) standing and looking down in 45 degrees (1.5 m distance). The experimental results revealed significant effects by the display type, rendering style and viewing angle. E.g. users tended to overestimate the object size and took longer to complete the task, when the small hand-held display is used. We believe the findings can serve as one guideline for developing effective AR applications.