In this study, an experiment was carried out to specify the colour appearance of a real room, which was painted with the same colour on four walls except the ceiling and floor. Two colour matching methods involving small colour patches were used to describe the colour appearance of different parts of one wall under two light sources (D65 and CWF). The results were used to model the size affecting colour appearance. It was found that there were changes of perceived lightness and chroma when the physical size of the same colour sample changed from a small colour patch to the wall of a room. However the perceived hue was unchanged when the physical size changed. These size effects were found to be independent of the matching methods and light sources used. Finally a model was derived.