In this work, the effect of temperature, exciton bandwidth, and size on the photoluminescence spectra of defect-free two-dimensional herringbone aggregates of pi-conjugated oligomers such as oligophenylene vinylene and oligothiophene is investigated theoretically. The model is based on exciton-phonon coupling in two-dimensional herringbone lattices with the exciton deriving from the lowest optical (1A(g)-->1B(u)) transition and the phonon from the most strongly coupled intramolecular vibrational mode with frequency omega(0). Simple analytical expressions are obtained for the line strengths of the emission origin (0-0) and first replica (0-1) as a function of the number of molecules comprising the aggregate, N, the free exciton bandwidth, W-D, and the temperature, T. At a given temperature, the 0-0 emission intensity initially scales as N/N-th, where N-th is the superradiant threshold number, but eventually converges to N-T/N-th, where N-T is the size independent thermal coherence number. N-T is inversely proportional to temperature and proportional to the exciton band curvature (omega(c)) near the band bottom; N-T=1+4piomega(c)/k(b)T. In striking contrast, the 0-1 line strength is relatively insensitive to temperature and size, but scales as the inverse square of W-D+omega(0). The insensitivity of the first replica to the exciton coherence number makes the ratio of the 0-0 to 0-1 line strengths a measure of the exciton coherence number. The ratio can be used to test for crystal purity. Comparison to experiments on thin films of quaterthiophene shows that the thermal coherence size is given by N(T)approximate to1+450/T (K) and that superradiance, which requires N-T>N-th, can only be observed at temperatures less than 1 K. (C) 2004 American Institute of Physics.