We show that the aspect ratio and the size of particles at surfaces can be estimated with photoelectron emission microscopy when both linear and nonlinear processes are utilized. As the width of the particles is known from regular photoemission microscopy, a complete determination of the particles’ dimensions becomes possible by two-photon photoemission microscopy. Here, the light diffraction pattern of the illuminating light around the particles is emphasized by the nonlinear dependence of the photoemission yield on the electric field components at the surface. This allows the quantitative measurement of the aspect ratio of the particles. The results are in agreement with theory and atomic force microscopy measurements.