Processing plays an important role in determining the microstructure of silicon nitrides which in turn influences the mechanical properties, such as hardness and toughness. Sintering aids are an important processing parameter. The influence of the chemistry of sintering aids on properties of silicon nitrides is a well-explored subject. Here the size of sintering aids used and its impact on microstructure and mechanical properties is explored. Specifically the use of nano-scale versus micron-scale sintering aids is examined. Microstructures and mechanical properties for silicon nitrides hot-pressed with nano-sized sintering aids are compared with a reference silicon nitride hot-pressed with micron-sized sintering aids. Hardness and fracture toughness are determined at room temperature using hardness indents. Grain size and aspect ratio distributions are determined for the two silicon nitrides and their impact on mechanical properties are examined. Toughening behavior is studied by experimentally determining R-curves. Both toughness and toughening (R-curve) behavior are shown to improve with the use of nano-scale sintering aids.