From architectural practice it is known that the perceived colour of large painted facades viewed outdoors at a distance from the house differs from the perceived colour of a small colour card placed directly onto it to perceptually match its colour. Authors believe that the explanation may be – at least partially – the so-called “colour size effect”. To study this, laboratory experiments have been carried out with controlled viewing conditions. Significant shifts of three (red, yellow and green) of the four unique hues were found on uniform self-luminant stimuli of a colour CRT monitor when the stimulus size changed from 10° to 120°. The mean overall hue difference was ΔH* = 2. The extent of the colour size effect varied among the 7 observers. Hue differences of the painted facades had a similar order of magnitude as the hue differences in the present study.