Improved toughness of heat-cured epoxies is readily achieved through chemical incorporation of a flexible moiety that phase-separates from the epoxy matrix during thermal cure. Many commercial flexibilizers are inherently polydisperse, which yields dispersions composed of flexibilizer chains of variable length. In this work, we examine the effect of flexibilizer polydispersity on the morphology and stress relaxation behavior of a commercial epoxy. This is achieved by systematically varying the composition of binary and ternary flexibilizer blends of monodisperse acrylate-terminated urethanes differing in molecular weight. Field-emission scanning electron micrographs of fracture surfaces permit quantification of dispersion sizes as a function of blend composition. Tensile stress relaxation data from three-point bend tests performed under isothermal conditions are analyzed in terms of a biexponential model to discern fast and slow characteristic relaxation times.