Flying insects chase prey and mates, skillfully exhibiting their ability to detect, track and pursue a moving target within a complex visual environment. This is a challenging task, even for the most sophisticated vision systems. Using electrophysiological techniques we record from visual neurons in the insect brain likely to underlie this pursuit behavior. These neurons respond to the presentation of small moving objects, even in the presence of background clutter. One such neuron, the centrifugal small target motion detector (CSTMD1), exhibits additional higher-order properties that may underlie a simple form of visual attention. This neuron's response is inhibited by the simultaneous presence of a second `distracter' target moving within a diffuse region of the excitatory receptive field. Intriguingly, a second distracter target moving in the visual field of the opposite eye completely suppresses the response of CSTMD1. In this study, we present distracter targets of varying sizes and determine whether the strength of these inhibitory interactions is itself a size selective phenomenon. We model CSTMD1's response to two-target stimuli presented within the excitatory hemifield as the summation of two size selective inputs, but inhibitory input from the other side as the difference. A further model elaboration includes half-wave rectification preceding the inhibition from the other eye, which results in a simple mathematical formulation encapsulating the response of CSTMD1 to both single and two target stimuli.