I like the idea of 20g as a cut-off for nanos, and 2.5g--->are pico reefs. It would be neat to make a poll and see the results. Id be willing to bet 15g-20g is the most popular choice for nano cutoff
From my own perspective, 10 gallon seems large. There are many more 5gallon and smaller reefs/planted tanks in the last two years than ever before in the hobby! They are starting to seem commonplace--I think the cutoff will get smaller in time.
small old reef tank:
the history of pico reef biology