  We experimentally study collective decay of an extended disordered ensemble
of $N$ atoms inside a hollow-core fiber. We observe up to $300$-fold enhanced
decay rates, strong optical bursts and a coherent ringing. Due to
inhomogeneities limiting the synchronization of atoms, the data does not show
the typical scaling with $N$. We show that an effective number of collective
emitters can be determined to recover the $N$ scaling known to homogeneous
ensembles over a large parameter range. This provides physical insight into the
limits of collective decay and allows for its optimization in extended
ensembles as used, e.g., in quantum optics, precision time-keeping or waveguide
QED.
