I call my 2ft cube my "big" tank because it's the biggest tank I've ever had, but I wouldn't describe it as huge or massive.
It's not always about the tank (except when stocking big fish), it's about how you use it. Many times I've seen nano tanks that look heaps better than monster tanks.
IMO:
-Less than 50L is nano
-50L-150L is average
-150L-250L is medium
-250L-400L is big
-400L-1000L is huge
-1000L+ is massive
Exceptions to those of course 8)