All known small Solar System bodies have a diameter between 1 m and a few hundreds of kilometers. Like all Solar System bodies, asteroids can be passive sources of high-energy gamma rays, which are produced when energetic charged cosmic rays impinge on their surfaces. Since the majority of known asteroids lie in an orbit between the orbits of Mars and Jupiter (known as the Main Belt), we expect them to produce a diffuse emission close to the ecliptic plane. In this work, we propose a model of the gamma-ray emission from the asteroids, written in terms of their size distribution. We show that the data collected by the Large Area Telescope (LAT) onboard the Fermi satellite can provide a way to constrain the asteroids gamma-ray emission model and, in turn, their size distribution.