How do I choose subwoofer size — 10, 12 or 15 inch?

Match the sub to the room's volume and how you use it, not to enthusiasm. Twelve inches is the sensible middle for most dedicated rooms; smaller and larger both have a proper place.

What driver size actually buys you is displacement, which is driver area times excursion. A bigger cone moves more air with less travel, so at high output it stays cleaner and reaches lower. That is why 15-inch and larger drivers, often in ported cabinets, suit big rooms and cinema levels: they deliver depth without strain. In a small room, the same sub is idling, and you paid for headroom you never use.

Compact sealed 10-inch subs suit smaller rooms and music-first listening. Sealed cabinets roll off more gently, which tends to sound tighter and integrates well with bookshelf speakers, and they are easy to hide. What they give up is deep output at high volume. Turn a film up in a large hall and a small sealed sub runs out of excursion early, which you hear as bass that goes flat and slightly distorted.

Twelve inches, sealed or ported, is the sweet spot for most 5.1 theatre rooms in the 150 to 250 square foot range. Enough displacement for cinema, still manageable in a living room.

Now the caveat that matters more than any of the above: driver size alone does not tell the story. Cabinet design, amplifier power and how the sub is tuned matter as much, so a well-engineered 10-inch can outperform a cheap 12-inch, and a poorly designed 15-inch is just a large box. Compare within a manufacturer's range using their published specs rather than across brands by cone diameter. A driver is not a promise; it is a component.

Indian rooms add one wrinkle. Marble and tile floors and glass fronts make room modes more pronounced, so evenness (placement, and sometimes two subs) matters as much as raw size.

Tell us the room dimensions and whether it is movies-first or music-first. Sub selection is one of the easiest things to get right with that information, and one of the most common things to get wrong without it.