Size effects are effects which might occur, if the dimensions of a forming process are scaled up or down. They might enable or disable the application of a process in the micro range. Based on the systematic order of size effects, which defines density, shape and structure effects, one example for each group is given. A density effect, which occurs in Tiffany structures, explains the changes in forming behavior of foils with respect to the forming limit diagram. The feasibility of a new heading process only in the micro range is due to a shape effect, driven by the surface energy. The changes in the tribology in deep drawing by a structure effect, known as closed an open lubricant pocket model, can be explained only if one takes the temperature dependence of the viscosity of the lubricant into account.