Look. It's real simple.
If you like low tuned songs, and you go with too small of port area, you'll hear a nasty huffing noise.
I've heard this noise from 4 18's on a 13x44 port (yes...)
The rule of thumb of 12-16 didn't come around on accident. You can go smaller but you'll have to take airflow into mind and think about it HARD. Kerf bends, flare ends, route edges.