This study aims to optimize the nozzle channel geometry and manufacturing parameters to obtain polymer parts with the required dimensional accuracy. For this purpose, samples with 30 strips were printed with an FDM printer. Each strip was measured at 4000 points using image processing method. To investigate the effect of the manufacturing parameters, a linear regression model was created based on the experimental data. The numerical model was created and validated comparing with the experimental results, and optimum nozzle channel geometry was obtained using the validated numerical model. Nozzles with the optimal channel geometries were manufactured and new samples were printed for comparison. Based on the experimental results, the strip width increased along the printing direction and ABS showed more swelling than PLA and CPE. The effects of feed rate, temperature, and measurement position on strip widths were evident for all materials. The results showed that the anticipated strip widths can be obtained for all materials using the optimized nozzle channel geometries. Thus, the dimensional accuracy of the 3D-printed parts can be improved using the optimized nozzles.