The grain-to-feature size ratio in micro-forming processes is predicted to have a vital impact on the material behavior in addition to the well-known effect of the grain size itself as manifested by the Hall-Petch relation. In this study, the "size effects" on the material flow curve of thin sheet metals under hydraulic bulge testing conditions were investigated. The ratio of the sheet thickness to the material grain size (N = t0/d) was used as a parameter to characterize the interactive effects between the specimen and the grain sizes at the micro-scales, while the ratio of the bulge die diameter to the sheet thickness (M = Dc/t0) was used to represent the effect of the feature size in the bulge test. A systematic approach for determining the flow curve of thin sheet metals in bulge testing was discussed and presented. The results of the bulge tests at different scales showed a decrease in the material flow curve with a decrease in N and/or M values. New material models were developed to explain the changes in the material flow curve caused by the size effects. The explanation and prediction of the flow curve based on these models were shown to be in good agreement with the bulge test results in this study and in the literature.