  The size of the Cooper pair $\xi_{\rm pair}$ is one of the basic
characteristics of a superconductor, but it is not possible to measure it
directly. It might be argued that $\xi_{\rm pair}$ can be determined from the
value $\xi_{\rm slope}$ extracted from the measurement of the slope of $H_{c2}$
close to $T_c$. Taking into account both pair-conserving and pair-breaking
scattering on impurities within the recently developed theory of Dynes
superconductors, we perform an explicit calculation of $\xi_{\rm pair}$ and
$\xi_{\rm slope}$. We show that the two quantities agree only in clean
superconductors. In particular, when the pair-breaking disorder approaches the
quantum critical point, $\xi_{\rm pair}$ stays finite, whereas $\xi_{\rm
slope}$ diverges.
