  We consider a definition of the QCD running coupling constant $\alpha(\mu)$
related to Wilson loops of size $r{\times}t$ with arbitrary fixed $t/r$. The
schemes defined by these couplings are very close to the $\overline{\rm MS}$
scheme (i.e.\ the one-loop perturbative correction to the coupling is small)
for all values of $t/r$; in the $t/r\to\infty$ limit, the ``$q\bar q$ force''
scheme is recovered, where the coupling constant is related to the
quark-antiquark force. We discuss the possibility of applying finite-size
scaling techniques to the Monte Carlo evaluation of $\alpha(\mu)$ up to very
large momentum scales. We propose a definition of correlation length, also
related to Wilson loops, which should make such a computation feasible.
