Most clownfish you will find in nano tanks are percs, ocellaris, and the genetic defects of them. A good switch are the orange or pink skunk clownfish.
Most clowns live in a natural environment where they don't go much further than 10" from their host, so, tank size is likely not important to them.
I have 3 sets in nano's around my house - one set in a 24g aquapod and 2 sets in 16g acrylic nano's. I have a 4th pair from an old friend in a 34g solana (gold striped maroons) but are much larger fish (5" for the female).
I would guess they are marketing and using the whole "nano" angle.