I'm looking at the spectrum of apertures that are available to today's amateurs, not ease of use or other factors. At most any major star party, an 18" truss-tube Dob hardly draws any attention any more ...
True. However, star-party attendees aren't necessarily typical of amateur astronomers in general. When writing an observing article for the broader public, I feel safe in calling a 16-inch scope big.
I am comfortable calling a 12.5 inch scope big/large. With dark skies, with a skilled observer, a 12 inch class scope can take you a long ways.
At a star party an 20 inch or 25 inch might be common place but that star party might be the only time during the year that the scope actually sees the night sky. People show their equipment at star parties but it is likely they do not have the opportunity to use a such a large scope at home. If I am like most other owners of larger telescopes, and a informal internet survey several years ago agreed with me, most have scopes that are more manageable that get more use. I am fortunate in that regard because I have a place to keep my larger scopes where the skies are reasonably dark but I still use the 12.5 inch and the 16 inch more than the 25 inch.
The purpose of this thread is help novices understand the ranking system used by authors when they discuss what is possible with small, medium and large scopes.. In that context, Tony is the one with the most experience. When I read such lists, generally it seems to me, the objects reserved for large scopes can be observed with a 12.5 inch if the conditions are favorable.