When a line extends beyond the width of an aperture, its direction of motion cannot be detected correctly. Only the component of motion perpendicular to the line is detectable (aperture problem). Early visual areas face the same aperture problem because receptive field sizes are relatively small. The susceptibility of early visual areas to the aperture problem opens an opportunity to measure the aperture width of a receptive field psychophysically that can be used to estimate the receptive field size. We found an already established visual illusion (the rotating tilted lines illusion or RTLI) can be used to measure the aperture size and hence estimate the receptive field size. To estimate the receptive field size, we conducted a psychophysical experiment in which the radii and tilted line length of RTLI were systematically changed. Our psychophysical estimation of receptive field size strongly corresponds with the previous measures of receptive field size using electrophysiological and fMRI methods.