  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 for all values of $t/r$; in the $t/r\to\infty$ limit, the ``$q\bar q$
force'' scheme is recovered. We propose a definition of correlation length,
also related to Wilson loops, which can be applied to the Monte Carlo
evaluation of $\alpha(\mu)$ up to very large momentum scales by use of
finite-size scaling techniques.
