I think a 20g is pretty much the standard for breeding clownfish. Some breeders even go down as far as 10g. Most tanks in aquaculturing facilities have overflow standpipes and central filtration systems. The broodstock tanks only have the clownfish and three ceramic tiles and nothing else. The less distractions for the fish, the better. If you feed them well and keep them bored, eventually they will mate.
Make sure you have all of the proper tools and live cultures ready when your clownfish spawn for the first time. Read a few good books and know what you are getting into before endeavoring on this journey.
Aquaculturing is really, really, fun and very addictive. lol.