  We study the stabilization of all closed string moduli in the T^6/Z_2
orientifold, using constant internal magnetic fields and 3-form fluxes that
preserve N=1 supersymmetry in four dimensions. We first analyze the
stabilization of Kahler class and complex structure moduli by turning on
magnetic fluxes on different sets of D9 branes that wrap the internal space
T^6/Z_2. We present explicit consistent string constructions, satisfying in
particular tadpole cancellation, where the radii can take arbitrarily large
values by tuning the winding numbers appropriately. We then show that the
dilaton-axion modulus can also be fixed by turning on closed string constant
3-form fluxes, consistently with the supersymmetry preserved by the magnetic
fields, providing at the same time perturbative values for the string coupling.
Finally, several models are presented combining open string magnetic fields
that fix part of Kahler class and complex structure moduli, with closed string
3-form fluxes that stabilize the remaining ones together with the dilaton.
