Smaller than a 10 gallon to be sure.
Often requires some unusual equipment or creative thinking, though that has changed a bit with red sea's and other new products. The fish world is designed for 10 gallons or larger, not smaller. Lights, filters, CO2, substrate, plants usually carried and fish usually stocked are, in 95% of the cases, too big for anything under 10, and certainly 5 gallon tanks.
The tanks are often DIY or originally designed for other purposes (glass vases), though this is changing. Until CF lighting, the smallest floros were 12". Even now CF lights are usually 7" or so long so many people have to resort to desk lamps with 5000k or 6500k replacement bulbs.
Nanos, to me, are about the creativity required as much as anything. It's a very DIY sized tank, just as tanks over 250 (or so) gallons will often be. It's a set-up where you are on the fringe of what the equipment is designed for and what most common knowledge is based on.
How's that for something a little more explicit than "smaller than 12" on a side"?