  We present a relativistic and model-independent method to derive
structure-dependent electromagnetic finite-size effects. This is a systematic
procedure, particularly well-suited for automatization, which works at
arbitrarily high orders in the large-volume expansion. Structure-dependent
coefficients appear as zero-momentum derivatives of physical form factors which
can be obtained through experimental measurements or auxiliary lattice
calculations. As an application we derive the electromagnetic finite-size
effects on the pseudoscalar meson mass and leptonic decay amplitude, through
orders $\mathcal{O}(1/L^3)$ and $\mathcal{O}(1/L^2)$, respectively. The
structure dependence appears at this order through the meson charge radius and
the real radiative leptonic amplitude, which are known experimentally.
