Measurements of critical temperature T-c in superconducting YBa2Cu3O6.9 films with reduced long-range structural order show the validity of the empirical scaling relation Delta T-c proportional to r(c)(-2) between disorder-induced reduction of T-c and structural coherence length r(c) in the ab-plane. This result is quantitatively explained by the disorder-induced confinement of the charge carriers within each ordered domain of size r(c). Our analysis of the data based on this picture enables us to precisely determine the Ginzburg-Landau superconducting coherence length in the ab-plane, xi(a)b = 1.41 +/- 0.04 nm.