Well, they seem to happen on the average once every million years per
galaxy, so they are hardly uncommon. But they are not galaxy-sized effects,
like the antimatter fountain. Since they vary over time scales as short as
a millisecond, they can't be bigger than a light-millisecond across, i.e.
3000 km. I just did a back of the envelope calculation (well, in my head,
actually) and determined that to get all the gamma rays from a burst into a
volume that small, they would have to be compressed to the density of lead.
Cool, huh?
--CarlF