  We study heterotic ground states in which supersymmetry is broken by coupling
the momentum and winding charges of two large extra dimensions to the R-charges
of the supersymmetry generators. The large dimensions give rise to towers of
heavy string thresholds that contribute to the running of the gauge couplings.
In the general case, these contributions are proportional to the volume of the
two large dimensions and invalidate the perturbative string expansion. The
problem is evaded if the susy breaking sectors arise as a spontaneously broken
phase of N=4 -> N=2 -> N=0 supersymmetry, provided that N=4 supersymmetry is
restored on the boundary of the moduli space. We discuss the mechanism in the
case of Z_2 x Z_2 orbifolds, which requires that the twisted sector that
contains the large extra dimensions has no fixed points. We analyse the full
string partition function and show that the twisted sectors distribute
themselves in non-aligned N=2 orbits, hence preserving the solution to the
string decompactification problem. Remarkably, we find that the contribution to
the vacuum energy from the N=2 -> N=0 sectors is suppressed, and the only
substantial contribution arises from the breaking of the N=4 sector to N=0.
