The HST team has suggested that they may be as large as several hundred metres, although this number was revised down in their press release. The ESO press release speaks of a few tens of metres. I have been arguing that they could be as small as a few metres across. First, it is very hard indeed to estimate the diameter of the nucleus of a comet.
Cometary nuclei are very dissimilar.
One can estimate the activity of the comet directly from the brightness as the brightness is just a function of how much dust and gas has been released. Changing brightness levels reflect changes in the comet's activity. The reason is that the relation depends on several factors, including the comet's composition and the fraction of the surface of the nucleus that is active. For very old comets the fraction of active area may be very low. New comets though tend to be active, at least whilst distant from the Sun, over almost all the surface of the nucleus.