Individual platelets in nacre-like ceramics are able to locally withstand stress levels many times larger than the strength of larger specimens made of the same materials. This size effect, usually reported as being relative to the size of pre-existing defects, is described by considering both stress and energy failure conditions. We show that there is a characteristic length, defined by the material's Young's modulus, fracture energy, and strength, above which failure is governed by a stress criterion and below which energy considerations are dominant. Applying the coupled criterion to three-point bending of a single alumina platelet allows the identification of the platelet strength and fracture energy. The proposed approach also allows capturing the decrease in failure stress due to a surface defect accounting for its shape and size.