Emission size distributions of primary anthropogenic aerosol particles are often prescribed in size-resolved global aerosol-climate models. They directly affect the simulated aerosol size distributions and lifecycle, especially near emission source regions. However, there is a large uncertainty in the assumed particle sizes in the literature. Here we investigate the impact of changing emission particle size distributions on aerosols and their radiative forcing in the Energy Exascale Earth System Model (E3SM) atmosphere model version 2 (EAMv2). Based on assumed emission sizes in the literature, we selected different particle sizes that determine the emission size distribution of black carbon (BC), primary organic matter (POM), and sulfate from different emission sectors. A reduction in BC/POM anthropogenic emissions size in E3SM leads to an increase in simulated aerosol number concentrations but a decrease in particle sizes in the primary carbon mode and accumulation mode. These changes become more pronounced with stronger perturbations. With the smallest assumed emission size, as suggested in the literature (an 18-fold increase in number emissions), EAMv2 simulates substantial changes in the simulated aerosol lifecycle and anthropogenic forcing. On the other hand, with larger assumed emission sizes for anthropogenic BC/POM, the aerosol number concentrations near source regions are significantly decreased, although the anthropogenic aerosol forcing is almost the same as in the reference simulations (with default emissions). The impact of emission sizes is also investigated for wildfire aerosol emissions and for primary sulfate from industry and energy sectors. Our results suggest that it is vital to constrain the uncertainty in the emission size distributions for primary emissions.