Sub size for 2 channel music in large room

I have a room 20x12 foot open on 2 sides, and ceilings are 16'.
What size subwoofer should I get for this size room? 12"? 15"? I heard small subs are better for music. Is this true even in a large room?
1st things first, your room is not large.

As a general proposition the smaller the driver the faster it can respond to changes (i.e. tighter bass). The smaller the driver the easier it is to integrate at a higher crossover lever. A good sub with a 12" woofer is usually easier to use than one with a 15 inch but won't go quite as low. A lot depends on the bass quality/quantity of the main speakers.

For example, if your main speakers have great bass down to say 40hz and you just need the bottom octaves you could use a 15" sub, but if your main speakers need to be rolled off much higher up you would probably be better off with a 12", or even better yet a modle with two smaller drivers (potentially even faster w/same volume capabilities.)
The generalization the smaller woofers are more articulate is fairly accurate. Another thing to keep in mind is that cheap subs do more things wrong than they do right. There are any number of subs that use low quality drivers and amplifiers that just sound muddy and distorted.

Best to demo in home if possible.

Subs have a great deal of interaction with the room. If your room and placement within the room do not work well with a specific sub that has proven to be good elsewhere you are still left wanting. High-output, highly adjustable subs with multiple smaller woofers is what I prefer.

Good luck!
My experience is that LF in small drivers (usually boxed in thin, tall elegant floorstanders) sound boomy and distorted at the low end of the bass response when compared to a very high quality 12" or 15" woofer.

A high quality large bass unit requires a huge magnet and is much more expensive to build than a little 6" woofer; not only is the larger magnet expensive but also the design requires a separate mid range driver rather than a simple cheaper two way design. A large woofer can't do the all important mid range very well even if it excels at the ultra LF. A small 6" driver can make a reasonable attempt at ultra LF and is well suited to the all important mid range.

I suspect that cost cutting to meet price points by manufacturers on these speakers with larger drivers may explain the generalization of "tighter" bass found with much smaller (and cheaper) bass/mid drive units in the ever-so-popular thin and tall speakers. At the same price point the small driver (cheaper design) may win out. This logic means that most manufacturers prefer to add several cheap 6" woofers to boost LF and make a very competitive alternative to a very costly 15" woofer design. In response, a manufacturer's design for a large woofer often uses a very cheap and poor quality bass driver in order to compete with the above price point; this means the design does not yield the expected LF improvement from the larger woofer (even though physics favors the large woofer).

In essence a small bass/mid woofer in a cost effective two way design will be better than a large bass woofer in a cost effective three way.

However, a very high priced, high quality large woofer design will generally sound better at LF than a very high priced high quality little woofer design. Although the cost of building a high quality large woofer design is obviously much more than the popular smaller woofer speaker design.

The logic for a sub wooofer is even clearer.....a large expensive woofer design will beat a small expensive woofer design hands down...simply because the physics heavily favors larger drivers for low distortion ultra LF.

In mid price level speakers ($5,000 and under), I would tend to go for small (6" or 8") woofers over large ones for better bass.

In a cost no object speaker system, I would never hesitate to go for large (12" or 15") woofers...the improvement is easily audible.

...just my two cents.
There is plenty of old discussion regarding one sub or two in the archives. Search on "need a subwoofer" and you should find a long thread.

The velodyne site has a chart of room szes measured by cubic feet. According to that, you need at least a 15". Two 12's = one 15 in output.

Contrary to what you usually hear, the Velodyne people say that their 15's and 18's are more accurate (all fast is good for) than the 10's and 12's for the DD range.