We quantitatively study the effect of growth-induced reduction of long range structural order on the superconducting transition in epitaxial YBa2Cu3O6.9 films. The corresponding reduction of structural coherence length r(c) is determined from the width of X-ray diffraction rocking curves. T-c measurements in the films give evidence for the validity of the empirical scaling relation Delta T-c similar to r(c,ab)(-2), where Delta T-c is the disorder-induced reduction of T-c and r(c,ab) is the structural coherence length in the ab-plane. To explain this algebraic law we propose a simple phenomenological model based on the disorder-induced localization of the charge carriers within each ordered domain of size r(c,ab). This picture enables us to precisely determine the Ginzburg-Landau superconducting coherence length in the ab-plane, and we obtain xi(ab) = 1.41 +/- 0.04 nm.