Considering the wide range applications of small metasurfaces in light focusing, in this paper effect of the included number of metaatoms and consequently metalens’s diameter on the focusing quality is considered. The results indicate that the number of metaatoms cannot be less than that is required to focus the incident light in a suitable place with an acceptable intensity. Metaatoms that are used in our proposed metalens in this paper are silicon cross-shaped on a silica substrate that provide a complete phase coverage between 0 to 2π. Simulation results show that for the designed metalens with focal length of 15 λ, the least numerical aperture value of 0.68 is required to have an acceptable quality of the focal spot. Using the required number of metaatoms, effect of the focal length on the focal depth is also studied. It is observed that for larger focal lengths, the focal depth will be longer.