  Protoplanetary discs are crucial to understanding how planets form and
evolve, but these objects are subject to the vagaries of the birth environments
of their host stars. In particular, photoionising radiation from massive stars
has been shown to be an effective agent in disrupting protoplanetary discs.
External photoevaporation leads to the inward evolution of the radii of discs,
whereas the internal viscous evolution of the disc causes the radii to evolve
outwards. We couple N-body simulations of star-forming regions with a
post-processing analysis of disc evolution to determine how the radius and mass
distributions of protoplanetary discs evolve in young star-forming regions. To
be consistent with observations, we find that the initial disc radii must be of
order 100au, even though these discs are readily destroyed by photoevaporation
from massive stars. Furthermore, the observed disc radii distribution in the
Orion Nebula Cluster is more consistent with moderate initial stellar densities
(100M$_\odot$ pc$^{-3}$), in tension with dynamical models that posit much
higher inital densities for the ONC. Furthermore, we cannot reproduce the
observed disc radius distribution in the Lupus star-forming region if its discs
are subject to external photoevaporation. A more detailed comparison is not
possible due to the well-documented uncertainties in determining the ages of
pre-main sequence (disc-hosting) stars.
