We have simultaneously measured, for the first time, the extension of the polymer backbone of a side-chain liquid crystalline polymer and the intensity of the 001 Bragg reflection, which gives the smectic order parameter Ψ as a function of temperature in the smectic phase. We have qualitatively demonstrated that the more the smectic phase is ordered, the more the polymer backbone is localized between the mesogenic layers. It is shown that the Landau theory allows us to relate the radius of gyration parallel to the magnetic field of the polymer backbone to the smectic order parameter. We also show that the Renz-Warner theory is suitable at low temperatures