Predicting the perceived brightness and lightness of image elements using color appearance models is important for the design and evaluation of HDR displays. This paper presents a series of experiments to examine perceived brightness/lightness for displayed stimuli of differing sizes. The number of observers in the first pilot experiment was 7, in the second and third pilot experiments was 6, and in the main experiment was 14. The target and test stimuli in the main experiment were 10∘ and 1∘ field of view, respectively. The results indicate a small, but consistent, effect that brightness increases with stimulus size. The effect is dependent on the stimulus lightness level but not on the hue or saturation of the stimuli. A preliminary model is also introduced to enhance models such as CIECAM16 with the capability of predicting brightness and lightness as a function of stimulus size. The proposed model yields good performance in terms of perceived brightness/lightness prediction.