That depends a lot on the tank mates and if they have an anemone or not.
I had an ocellaris pair in a flowerpot without anemone claiming an entire 750 liter tank its own private kingdom while a fully grown clarkii pair that lived in a 30 cm BTA only claimed about 20 cm around its anemone.
It was quite similar with the above ocellaris when I had them in an anemone in a smaller tank. They only claimed the anemone and its immediate surrounding and you could see that they felt extremely insecure as soon as they left that safety zone.
I see the same right now with my current percula pairs.
So if you want to keep your clownfish restrained to a defined range give them an anemone (ideally one of their natural hosts).
As one who does not subscribe to the Contadina style of fish keeping, for the average aquarist on this board, the guidelines I provided will be valid the vast majority of the time. Can you confine them to less space? Yes. Will it always work out? Not necessarily so. Especially if you choose one of the more aggressive species of clownfish.