anything over 15g and traditional hardware works pretty well, that's why I dont consider a 20g a nano, but under that size, things dont fit too well, and you have to get creative in both hardware and maint, as it's not a traditional approch.
In perspective, a 20g is 4x larger than my current nano, I also had a 20g long for quite a while, and it was nothing like a nano, the extra volume makes a huge difference, re-enforcing why I dont consider it a nano.
The only reason I included a 15g tall, is it's a odd size, really the cuttoff in my mind is anything over 10g, also people have been doing 20g reefs for years and years, nothing new there, however, 10g and smaller were not really popular until about 4-5 years ago.