The holographic microscopy technique is a strong contender for dynamic three-dimensional (3D) measurement of small particles (typically smaller than 5 microm) in microchannels. However, there is a big challenge to accurately measure the size of such small particles. The traditional hologram reconstruction method was numerically investigated. It is found that the error level, especially for the size measurement, is higher than expected, even in an ideal situation without consideration of noise. An alternative way based on Lorenz-Mie (LM) calculations was then presented. The intensity distribution of the fringe pattern on the particle hologram is directly used and compared with the ones calculated using an LM-based program. Various cases for particle sizes from 0.5 to 5 microm and recording distances from 5 to 500 microm are tested. The results show that the accuracy in particle sizing can be significantly improved.