The effects of along and transverse radar beam drifts of sporadic E (Es) field-aligned irregularities localized in the expected echoing region on mean Doppler velocity and spectral width are theoretically investigated in this article. We show that the mean Doppler velocity of the irregularities is not only governed by the along and transverse beam drift velocities, but also affected by the mean position and zonal extent of the organized plasma structure in the expected echoing region that can be determined by using IGRF2000. Detailed analysis indicates that the Doppler velocity nearly linearly proportional to the mean angular distance of the irregularities from the radar beam axis decreases with the increase of the horizontal dimension of the plasma structure. Theoretical results also predict that the Doppler spectral width of the Es field-aligned irregularities is not only the function of the transverse beam drift velocity, but also related to the zonal extent of the plasma structure. Quantitative estimation shows that the beam broadening spectral width induced by the geomagnetically zonal drift of the irregularities at the velocity of 150 m/s can be more than 9 m/s if the zonal dimension of the plasma structure located at height 105 km is 5 km. Therefore it suggests that the beam broadening effect caused by the drift of the field-aligned irregularities across the radar beam in the geomagnetically zonal direction may play a role in broadening the Doppler spectral width.