Identifying a target is more difficult when distracters are present within a zone of interaction around the target. We investigated whether the spatial extent of the zone of interaction scales with the size of the target. Our target was a letter T in one-of-four orientations. Our distracters were four squared-thetas in one-of-two orientations, presented one in each of the four cardinal directions, equidistant from the target. Target-distracter separation was varied and the proportion of correct responses at each separation was determined. From these the extent of interaction was estimated. This procedure was repeated for different target sizes spread over a 5-fold range. In each case, the contrast of the target was adjusted so that its visibility was constant across target sizes. The experiment was performed in the luminance domain (grey targets on grey background) and in the chromatic domain (green target on equiluminant grey background). In the luminance domain, target size had only a small effect on the extent of interaction; these interactions did not scale with target size. The extents of interaction for chromatic stimuli were similar to those for luminance stimuli. For a fixed target visibility, decreasing the duration of the stimulus resulted in an increase in the extent of interaction. The relevance of our findings is discussed with regard to a variety of proposed explanations for crowding. Our results are consistent with an attention-based explanation for crowding.