Here is a picture of 4 SVS subs, for size comparison for you...
The 2 on the left are SVS-PB12 Plus/2's, on the right is the PB12-Plus
On top is the SB12-Plus.
The Ultra13 is similar in size (in Between) the Plus/2 and the Plus.
Subs I mentioned, Paradigm Servo .v2, JL Audio F112 & F113 are inbetween the large subs and smallest sub, as would be the Velodyne DD-series. Any of these subs listed will be way more then enough for that medium size room, infact I would suggest that the SVS SB12-Plus would suffice as well... As its pounds, cleanly and clearly in my 1800 cu ft room, and has room to boot. Sealed is a good choice for music...
The other subs mentioned here in this thread are also good choices....