The effect on the coherence length of excitations in polysilane copolymers due to electron-donating (dimethylamino)phenyl (DMAP) side groups is investigated by time-resolved and steady-state fluorescence studies. In the case of poly(methylphenylsilane) the lowest excitonic state has been shown to delocalize over several tens of monomer units. Steady-state studies of the novel copolymers indicate three emitting species which can be assigned to three fluorescence lifetimes identified by time-resolved experiments. A fast lifetime of a few tens of picoseconds corresponds to the delocalized lowest excited state. A second delocalized excitation with a short lifetime and a third "self-trapped exciton" with a coherence length of approximately a single silicon bond are also proposed. The size of the delocalized excitations in the copolymers exhibits a strong dependence on the concentration of the electron-donating DMAP side group. The coherence length of the excitations can be predicted by a simple statistical treatment of the distribution of homogeneous chain segments terminated at each end by DMAP side groups. Consequently, a perturbation of the backbone states due to the electronic nature of the DMAP side groups is proposed to be the origin of the force which defines the size of the exciton.