To examine the spatial scale of the mechanism supporting the perception of motion-in-depth defined by binocular disparity cues, we measured stereomotion speed discrimination thresholds as a function of stimulus width using a 2IFC paradigm. Dynamic random dot stereogram bars, wherein new but perfectly binocularly correlated dot arrays are presented interleaved at a rate of 120Hz per eye, were displayed using ferro-electric shutter glasses on a 240Hz fast-phosphor monitor. Frame-by-frame manipulation of disparity simulated receding motion (standard retinal speed: 0.62 deg/s). We jittered the disparity starting point (±0.1 deg) and the stimulus duration (500-700 ms) to render the use of static disparity and total displacement cues ineffective. Stimuli ranged in vertical extent from 0.02 to 0.66 deg, but had a constant width of 7.3 deg to minimise and hold constant any effect of monocular half-occlusion artifacts. A background of static random dots allowed us to avoid visibility issues as the stimulus passed through zero relative disparity. Multiple interleaved staircases were used to measure speed-discrimination performance for each size condition, with thresholds computed using Probit analysis. For all three (two naäve observers tested, Weber fractions were strongly related to stimulus width. For the largest width, they were 19, 25, and 26% for the 3 observers. Performance decreased for smaller widths, becoming nearly random for widths below 0.08 deg. However, for standard random dot stimuli, which contain monocular motion cues as well as changing disparity cues, speed discrimination remained robust, even at the smallest width tested. The spatial resolution of the changing disparity mechanism supporting the perception of motion-in-depth is much more coarse than any monocularly based motion-in-depth mechanism.