The signatures of exciton relaxation in time-resolved fluorescence spectroscopy of molecular aggregates excited by a short pulse are expressed as an overlap of a Doorway wavepacket representing the exciton density matrix with a Window wavepacket which describes the time and frequency resolved detection. Transport and relaxation of the excitons are accounted for using the Redfield equations for the density matrix, and the complete temporal and spectral profiles of the excitation pulse and detection gate are incorporated using Wigner spectrograms. The spread in the off diagonal elements of the density matrix in the chromophore (real space) representation provides a natural measure of the relevant exciton coherence size.