Two-dimensional molecular dynamics computer simulation has been developed to model the compression of Langmuir films composed of spherical nanoparticles with arbitrary size distribution. We demonstrate that the usual assumption in the determination of interparticle potentials from the surface pressure vs area isotherms (i.e., monodisperse particles in perfect hexagonal order) leads to a systematic overestimation of the characteristic length of the interaction. On the basis of the results of the simulation, we propose a correction method to improve the traditional way of determining the interparticle potentials. We use the corrected particle-particle interactions to explore the correlation between the broadness of the size distribution and several structural parameters (decay length of pair-correlation function, global orientational order parameter, mean, and standard deviation of number of neighbors). Due to the uniaxial compression and the stiffness of the particulate layer, the surface pressure is not a scalar field. We investigate the effect of polydispersity on the anisotropy and the fluctuation of the surface pressure tensor in Langmuir films during uniaxial compression.