  In order to characterize the geometrical mesh size $\xi$, we simulate a
solution of coarse-grained polymers with densities ranging from the dilute to
the concentrated regime and for different chain lengths. Conventional ways to
estimate $\xi$ rely either on scaling assumptions which give $\xi$ only up to
an unknown multiplicative factor, or on measurements of the monomer density
fluctuation correlation length $\xi_c$. We determine $\xi_c$ from the monomer
structure factor and from the radial distribution function, and find that the
identification $\xi=\xi_c$ is not justified outside of the semidilute regime.
In order to better characterize $\xi$, we compute the pore size distribution
(PSD) following two different definitions, one by Torquato et al. (Ref.1) and
one by Gubbins et al. (Ref.2). We show that the mean values of the two
distributions, $\langle r \rangle_T$ and $\langle r \rangle_G$, both display
the behavior predicted for $\xi$ by scaling theory, and argue that $\xi$ can be
identified with either one of these quantities. This identification allows to
interpret the PSD as the distribution of mesh sizes, a quantity which
conventional methods cannot access. Finally, we show that it is possible to map
a polymer solution on a system of hard or overlapping spheres, for which
Torquato's PSD can be computed analytically and reproduces accurately the PSD
of the solution. We give an expression that allows $\langle r \rangle_T$ to be
estimated with great accuracy in the semidilute regime by knowing only the
radius of gyration and the density of the polymers.
