Reflection high-energy electron diffraction (RHEED) is so sensitive to surface morphology that it is difficult to separate the roles of instrument response and surface imperfection in the diffraction. To address this difficulty we have used MBE grown GaAs(001) as a test surface to study the angular dimensions of the diffracted beams. This is important if RHEED is to be useful as a quantitive probe of surface structure for in situ studies of crystal growth. The limitations placed by instrumental uncertainties on the maximum resolvable distance are estimated. Measurements of the angular length of the RHEED steaks versus angle of incidence are presented which show the changes expected from a combination of a uniformly broadened reciprocal lattice rod and an instrument limit due to angular uncertainties. Resolvable distances are obtained with RHEED that are much larger than those typically obtained with low-energy electron diffraction.