Some questions.
1. What are the dimensions of the SongSub?
The SongSub is 14" wide, 16" deep and 24" tall.
2. using a pair of subs with a pair of Birds how much space would be needed for each set?
That all depends on where you place the subs. Since you can adjust the phase of the subs, they can be placed almost anywhere (your sense of hearing cannot localize sources under about 150Hz or so).
3. Where in relation to the towers should the sub be placed?
Anywhere in the room that works is just fine. Think more in terms of evenly pressurizing the room with the subs. You can adjust the gain and phase of the subs to integrate them with the mains.
I hope this helps.
- Jim