LARGE RADII AND STRING UNIFICATION

We study strong coupling effects in four-dimensional heterotic string models where supersymmetry is spontaneously broken with large internal dimensions, consistently with perturbative unification of gauge coupl ings. These effects give rise to thresholds associated to the dual the ories: type I superstring or M-theory. In the case of one large dimens ion, we find that these thresholds appear close to the field-theoretic al unification scale similar to 10(16) GeV, offering an appealing scen ario for unification of gravitational and gauge interactions. We also identify the inverse size of the eleventh dimension of M-theory with t he energy at which four-fermion effective operators become important.