Physical constraints produce variations in the shapes of biological objects that correspond to their sizes. Bingham (1992) showed that two properties of tree form could be used to evaluate the height of trees. Observers judged simulated tree silhouettes of constant image size appearing on a ground texture gradient with a horizon. According to the horizon-ratio hypothesis, the horizon can be used to judge object size because it intersects the image of an object at eye height. The current studies investigated whether the locus of the horizon might account for previous results. Tree images were projected to a simulated eye height that was twice that used previously. Judgments were not halved as predicted by the horizon ratio hypothesis. Next the original results were replicated in viewing conditions that encouraged use of the horizon ratio by including correct eye height, gaze level, and visual angles. The heights of cylinders were judged inaccurately when they appeared with horizon but without trees. Judgments were much more accurate when cylinders also appeared in the context of trees