  Higher-dimensional theories provide a promising framework for unified
extensions of the supersymmetric standard model. Compactifications to four
dimensions often lead to U(1) symmetries beyond the standard model gauge group,
whose breaking scale is classically undetermined. Without supersymmetry
breaking, this is also the case for the size of the compact dimensions.
Fayet-Iliopoulos terms generically fix the scale M of gauge symmetry breaking.
The interplay with supersymmetry breaking can then stabilize the compact
dimensions at a size 1/M, much smaller than the inverse supersymmetry breaking
scale 1/mu. We illustrate this mechanism with an SO(10) model in six
dimensions, compactified on an orbifold.
