We report on a careful analysis of finite size effects on pressure measurements in Monte Carlo computer simulations of isotropic athermal solutions of flexible polymer chains by means of the repulsive wall method. We find finite size corrections to the pressure due to surface effects. These corrections are inversely proportional to the thickness of the simulation box, both if we keep the average density (in the canonical ensemble) or the chemical potential (in the grand canonical ensemble) constant in course of the preparation of the starting conformation. We propose a modification of the repulsive wall method which allows avoidance of these finite size effects and to estimate the pressure for an infinite system when running simulations in a finite box.