To test the hypothesis that perceived size of space depends on perceived depth, a relation between judged depth and size of space was derived. External spaces were judged from colour photographs by the magnitude estimation method. It was found that judged depth is a power function of physical depth and judged size of space a power function of physical size (rectangular area) or of judged depth times perceived width. The results indicate deviations from the derived relation and some explanations for these are discussed. Further research is needed to verify the proposed hypothesis.