  We estimate the significance of the prediction for the gauge couplings in the
MSSM with an underlying unification. The correlation between the couplings
covers only (0.2-2)% of the a priori reasonable region of the parameter space,
while the prediction for $\sin ^{2}\theta_{W}$ is accurate to 1.3%. Given that
agreement with experiment to such precision is unlikely to be fortuitous, we
discuss the profile of a string theory capable of preserving this level of
accuracy. We argue that models with a low scale of unification involving power
law running in the gauge couplings do not. Even theories with a high scale of
unification are strongly constrained, requiring the compactification scale of
new space dimensions in which states transforming under the Standard Model
propagate to be very close to the string cut-off scale. As a result {\it no}
new space dimensions can be larger than $10^{-14}fm.$
