Conventional material models cannot describe material behaviors precisely in micro/mesoscale due to the size/scale effects. In micro/mesoscale forming process, the reaction force, localized stress concentration, and formability are not only dependent on the strain distribution and strain path but also on the strain gradient and strain gradient path caused by decreased scale. This study presented an analytical model based on the conventional mechanism of strain gradient (CMSG) plasticity. Finite element (FE) simulations were performed to study the effects of the width of microchannel features. Die sets were fabricated and micro/mesoscale sheet forming experiments were conducted. The results indicated that the CMSG plastic theory achieves better agreements compared to the conventional plastic theory. It was also found that the influence of strain gradient on the forming process increases with the decrease of the geometrical parameters of tools. Furthermore, the feature size effects in the forming process were evaluated and quantitated by the similarity difference and the similarity accuracy. Various tool geometrical parameters were designed based on the Taguchi method to explore the influence of the strain gradient caused by the decrease of tool dimension. According to the scale law, the difference and accuracy of similarity were calculated. Greater equivalent strain gradient was revealed with the decrease of tool dimension, which led to the greater maximum reaction force error due to the increasing size effects. The main effect plots for equivalent strain gradient and reaction force indicated that the influence of tools clearance is greater than those of punch radius, die radius, and die width.