  It remains unclear what mechanism is driving the evolution of protoplanetary
disks. Direct detection of the main candidates, either turbulence driven by
magnetorotational instability or magnetohydrodynamical disk winds, has proven
difficult, leaving the time evolution of the disk size as one of the most
promising observables able to differentiate between these two mechanisms. But
to do so successfully, we need to understand what the observed gas disk size
actually traces. We studied the relation between $R_{\rm CO,\ 90\%}$, the
radius that encloses 90% of the $^{12}$CO flux, and $R_c$, the radius that
encodes the physical disk size, in order to provide simple prescriptions for
conversions between these two sizes. For an extensive grid of thermochemical
models we calculate $R_{\rm CO,\ 90\%}$ from synthetic observations and relate
properties measured at this radius, such as the gas column density, to bulk
disk properties, such as $R_c$ and the disk mass $M_{\rm disk}$. We found an
empirical correlation between the gas column density at $R_{\rm CO,\ 90\%}$ and
disk mass: $N_{\rm gas}(R_{\rm CO,\ 90\%}) \approx 3.73\times10^{21}(M_{\rm
disk}/\mathrm{M}_{\odot})^{0.34}\ \mathrm{cm}^{-2}$. Using this correlation we
derive an analytical prescription of $R_{\rm CO,\ 90\%}$ that only depends on
$R_c$ and $M_{\rm disk}$. We derive $R_c$ for disks in Lupus, Upper Sco, Taurus
and DSHARP, finding that disks in the older Upper Sco region are significantly
smaller ($\langle R_c \rangle$ = 4.8 au) than disks in the younger Lupus and
Taurus regions ($\langle R_c \rangle$ = 19.8 and 20.9 au, respectively). This
temporal decrease in $R_c$ goes against predictions of both viscous and
wind-driven evolution, but could be a sign of significant external
photoevaporation having truncated disks in Upper Sco.
