We report evidence for the reduction of long range lattice order caused by slight departures from the optimal growth temperature in fully doped (x≈0.9) YBa2Cu3O6+x films deposited by ion-beam sputtering on 〈001⟩ SrTiO3. We estimate the characteristic length of this disorder from the broadening Δ∂ of the 〈005⟩ X-ray diffraction rocking curve. The depression of superconductivity and normal conductivity scales as Δ∂ and disappears when the in-plane lattice coherence length rc~1/Δ∂ is larger than ≈10 nm