• Grain size is the dominant factor controlling resistivity in polycrystalline 3C-SiC with similar doping levels.
• A quantitative relationship log(ρ) = −1.93 + 8.67/d was established between resistivity and grain size.
• TEM, XRD, Raman spectroscopy, and EBSD were combined to characterize microstructure and stress.
• The findings enable quantitative control of electrical properties for polycrystalline 3C-SiC applications.