Nanoparticles of ∼55 nm in diameter have been found to significantly increase the fracture toughness of nanocomposites with a ductile polymer matrix. This paper presents a comparative study of the effectiveness of micrometer-sized and nanosized elastomeric particles in improving the fracture toughness and strength of polymeric adhesives. Particular focuses are on the effects of particle size, matrix ductility, and adhesive thickness on the shear strength and fracture toughness. The results reveal that for an epoxy adhesive cured with a J400 hardener, nanosized particles can produce nearly 18 times more increase in fracture toughness than what can be achieved using micrometer-sized particles at the same volume fraction of 5.0 vol%. This huge improvement of adhesives' fracture toughness by nanosized particles is similar to that observed in bulk nanocomposites, indicating that the superior toughening mechanism of nanosized elastomeric particles is equally effective in thin adhesives constrained by stiff adherends.