We consider meson finite size effects in mesonic atoms. A limit on the q.e.d. cut-off mass Λ is set by present data on muonic 206Pb at Λ > 1.44 GeV. This limit is comparable to recent high energy estimates and it can be improved upon. We indicate the importance of the pion form factor in extracting the strong π-nucleus optical potential. Finally, we discuss how a possible pion polarizability may be detected.