Measurement techniques for in-situ simultaneous measurements of particle size distributions and particle velocities using the dual beam laser Doppler velocimeter (LV) were investigated. This investigation concentrated on examination of the different signal characteristics of the LV for extraction of both particle size and particle velocity. These different signal components were evaluated not only singularly but also as sets of multiple simultaneous measurements to determine which characteristic, or combination of characteristics, provides the best measure of particle size. Different components of the LV signal were considered, but analysis concentrated on Doppler phase angle, visibility, and scatter-intensity because they show the greatest promise. Because LV signal characteristics are not monotonic functions of the particle size parameter, the resulting uncertainties introduced in the size-signal relationship were identified for the signals for both separate and simultaneous measurements. Uncertainties introduced by multiple particle indices of refraction were also evaluated. Finally, uncertainties resulting from particle trajectory variations in the spatially nonuniform LV probe volume were considered. No one signal characteristic or optical configuration was found which would minimize all uncertainties, but optimal configurations were identified for selected applications. Experimental verification and further optimization are recommended. The most promising instrument should be designed and tested to perform this validation, while allowing experimental comparisons of possible instrument configurations and developing instrument design concepts.