Precision machining is a critical aspect of modern manufacturing, enabling the production of components with extremely tight tolerances and high surface quality. One of the key factors influencing the performance and outcomes of precision machining is the size effect, which refers to the varying behavior of materials when the size of a workpiece or feature is reduced to a certain scale. Two key factors that contribute to dimensional effects in precision machining are tool radius and cutting depth. This chapter will focus on these two factors contributing to size effects in precision machining. It will discuss the impact of tool radius and cutting depth on size effects in micro-milling and ultra-precision fly cutting processes, respectively.