There are certain requirements for the mechanical properties of paper or paper-based materials during their service. However, existing experimental and theoretical studies of size effects indicate that the mechanical properties of specimens with different sizes are not identical. Therefore, it is necessary to clarify the size effect of their mechanical properties to ensure their safety. The kraft paper, which is easy to obtain and has high strength, is selected to carry out uniaxial tensile tests on with different sizes. And then the size effects of mechanical properties and their dispersions are determined. By analyzing the deformation characteristics and macro-microscopic damage features of different sizes specimens, the intrinsic mechanism of the size effect on the mechanical properties of kraft paper is revealed. Finally, a size effect model of mechanical properties for kraft paper is established. The study finds that the nominal tensile strength and nominal peak strain of kraft paper first increase and then decrease as the sizes increases. This is because the cut edge fibers cause the edges of the kraft paper specimens to have weaker stress and strain capabilities, and the proportion of weakened edges in small-sized specimens is relatively large, leading to an increasing size effect on the nominal tensile strength and nominal peak strain. In large-sized specimens, the proportion of weakened edges can be ignored, and the internal fracture process zone plays a dominant role, leading to a decreasing size effect on the nominal tensile strength and nominal peak strain. The established edge-internal fracture process zone size effect model can well describe the size effect on the nominal tensile strength and nominal peak strain of kraft paper. However, this model cannot capture the nonlinear characteristics of the increasing size effect stage for the nominal tensile strength and nominal peak strain.