Size effect on fracture properties of MEMS and traditional materials is detennined by the relative size of a testing specimen versus the material microstructure. In this study, size effect on quasi-brittle fracture is related to the length ratio determined by the fracture process zone (FPZ) and distance of a crack-tip to its nearest specimen boundary. It is shown that the tensile strength criterion applies if the specimen boundary is close to the crack-tip, and the fracture toughness criterion applies if the specimen boundary is away from the crack-tip. The specimen boundary influence reflects the dominant size effect mechanism, i.e. the interaction of the crack-tip FPZ with the specimen boundary. The boundary effect model proposed in the study is compared with the common size effect model emphasising exclusively the influence of specimen size, and the major difference is discussed.