In this paper, we demonstrate how polarization-resolved holography can be used to determine if a particle is spherical or not and to estimate the size information of nanoparticles. The T-matrix method is used to model the scattered light from both spheres and spheroids. A dual-view polarization-resolved imaging system is used in order to obtain polarization ratio angles (β1,β2). From the obtained β1 and β2, it is possible to estimate whether or not a particle is spherical. It is found that nonsphericity only has a minor effect up to around sizes of 120 nm, and for that range, a spherical approximation is valid. For larger particles, the orientation influences the polarization response greatly. The size of a nonspherical particle can be estimated from the polarization ratio angles. The upper limit we can estimate unambiguously is around 200 nm. Finally, the model is applied to experimental measurements of naturally occurring particles in purified water. From the measurements, it is possible to separate spherical from nonspherical particles and also give a rough estimate of the size.