We report the first quantitative measurements of the smectic penetration length $\lambda = \sqrt{K/\bar{B}}$, with K and $\bar{B}$ the curvature and compression elastic constants, for ferrosmectics, which are doped lyotropic smectics. We use a method based on the observation of defects created in a cell of varying thickness. In particular, we determine the evolution of λ with the volume fraction of doping particles, φ. The main experimental features are that λ is constant at small φ then decreases, and that this diminution is enhanced at large swelling of the phases. The results and their consequences on K and $\bar{B}$ are discussed ; they support and complement previous neutron scattering studies. We also observe edge dislocations with very small Burgers vector, that were not expected in such swollen phases. A microscopic model is proposed to evaluate λ from such defects.