  Gamma-ray bursts (GRBs) show a bimodal distribution of durations, separated
at a duration of ~2 s. Observations have confirmed the association of long GRBs
with the collapse of massive stars. The origin of short GRBs is still being
explored. We examine constraints on the emission region size in short and long
GRBs detected by Fermi/GBM. We find that the emission region size during the
prompt emission, R, and the burst duration, T$_{90}$, are consistent with the
relation R ~ c x T$_{90}$, for both long and short GRBs. We find the
characteristic size for the prompt emission region to be ~2 x 10$^{10}$ cm, and
~4 x 10$^{11}$ cm for short and long GRBs, respectively.
