Based on resolving power, a 10" Newtonian Telescope is the smallest telescope that can resolve the best resolution permitted by atmospheric seeing conditions. As I pointed out earlier, these conditions are few and far between. However, with adaptive optics the average seeing can be improved from 3.0 arc sec. to 0.7 arc sec., which can be resolved by a 6.75" telescope located beyond the atmosphere.
A 6.75" telescope only has the Light Gathering Power of
(6.75 * 25.4 / 7)$$² = 599.9.  A 10" telescope with adaptive optics has a Light Gathering Power of (10 * 25.4 / 7)$$² = 1,316.7.  While the 10" telescope's resolving power may have been reduced to that of a 6.75" telescope, it remains 2.2 times brighter than the smaller telescope, - whithout ever droping below the smaller telescope's resolving power.
This works for larger telescopes as well.  Thus a 30" telescope equipped with the same adaptive optics would only have the Resolving Power of a 6.75" telescope but it would retain its Light Gathering Power of 11,849.9, which is 19.8 times brighter than the smaller telescope.  We need to determine how much Light Gathering Power is required to detect a comet at a safe distance.