"Nano" is a frequently overused term in aquarium circles. People tend to exaggerate the smallness of their aquaria!
The true meaning of the term is "a billionth", so it means tiny, not just small.
I have a 26 litre freshwater planted tank, which I refer to as a "nano", but I should probably refer to it as a "micro" (I've never seen that term used, but we should use it instead of the hyperbole of "nano"). In fact, from this day on that's what I'll do. 26 litres is indeed fairly small, but it's not really tiny.
I also have a four litre planted bowl, which has some trumpet snails in it and might be ready for a Nerite snail. It's probably too small for any other fauna, although once it has matured for a few more months I might try netting a cherry shrimp or two from my other tank to see if they can survive in the bowl. I have referred to this as a "pico", but I now think I'm guilty of exaggeration as well. This bowl is really "nano". Some people are setting up whisky tumblers, and that is truly "pico".
The rules for classifying saltwater tanks are justifiably different because it's very much more difficult to keep a very small saltwater tank than freshwater. 26 litres would indeed be incredibly small for a saltwater tank and would definitely deserve the term "nano".