  Most stars form in a clustered environment. Therefore, it is important to
assess how this environment influences the evolution of protoplanetary discs
around young stars. In turn, this affects their ability to produce planets and
ultimately life. We present here for the first time 3D SPH/N-body simulations
that include both the hydrodynamical evolution of the discs around their natal
stars, as well as the dynamics of the stars themselves. The discs are viscously
evolving, accreting mass onto the central star and spreading. We find
penetrating encounters to be very destructive for the discs as in previous
studies, although the frequency of such encounters is low. We also find,
however, that encounter influence the disc radii more strongly than other disc
properties such as the disc mass. The disc sizes are set by the competition
between viscous spreading and the disruptive effect of encounters. As discs
spread, encounters become more and more important. In the regime of rapid
spreading encounters simply truncate the discs, stripping the outer portions.
In the opposite regime, we find that the effect of many distant encounters is
able to limit the disc size. Finally, we predict from our simulations that disc
sizes are limited by encounters at stellar densities exceeding $\sim 2-3 \times
10^3 \ \mathrm{pc}^{-2}$.
