i think it depends on the tank. i don't consider anything with a sump/fuge a nano, really. nano's seem to me to be an all-in-one-type of thing. a nano to me seems a tank the size of a sump, turned into a DT (and under, of course)
i wouldn't consider anything over a 20 a nano, unless it's the all-in-one tank, which those will probably get bigger in the future, too. so, 29 and under for the bio-cube types, 20 and under for other.
personally, you can call it whatever you want, i don't care but, if i had one, those are the guidelines i'd go by. :haha: