  Structural phase separation in the form of partially disordered stripes, with
characteristic length scales in the nanometer range, is observed for
superconducting BaPb$_{1-x}$Bi$_x$O$_3$. The evolution of the superconducting
coherence length with composition relative to the size of these stripes
suggests an important role of the nanostructure in determining the shape of the
superconducting dome. It is proposed that the maximum $T_c$ is determined by a
kind of "optimal inhomogeneity", characterized by a crossover from an
inhomogeneous macroscopic superconductor to a granular superconductor for which
phase fluctuations suppress $T_c$.
