Signatures of the exciton coherence size, which controls the nonlinear opti
cal response and luminescence of conjugated systems, in the electronic dyna
mic structure factor S(q, omega) are calculated. We find that for small mol
ecules the momentum dependence of the lowest exciton resonance is purely ge
ometric, reflecting the molecular size rather than a universal exciton size
, as suggested recently. For long chains the q dependence is determined by
the interplay of the exciton size and the bond-alternation length scales.