The influence of carbon nanoparticle morphology in epoxy on the mechanical properties is investigated regarding the size effect. Three types of particles are used, representing spherical, tubular and layered morphologies. For neat and carbon black modified epoxy, a clear size effect of increasing true failure strength with decreasing volume is found. Carbon nanotube modified epoxy shows high potential for strength increase, but dispersion and purity are critical. Few layer graphene particles are larger than statistically distributed defects in the epoxy and initiate cracks, counteracting any size effect. Different toughness increasing mechanisms on the nano- and micro-scale depending on particle morphology are discussed.