Since the sound propagation goes through the center of each room brush, you might get odd results if they are too large (e.g. footsteps of a nearby guard might suddenly sound very far away when he traverses into the next room brush). I don't know about exact guideline dimensions. It's best to test the sound propagation yourself and adjust any areas that behave oddly.
No room brush should overlap the center of another one. Otherwise you can place room brushes as you want, even through solid walls (e.g. to simulate a water pipe behind a wall).
To see if the connections between room brushes work correctly, select a room brush and click on Show All or Show Sel in the bottom menu. Show All displays all room connections as blue lines. If you select Show Sel, only the connections of the currently selected room are displayed. Click on Show Sel/Show All again to make the lines disappear.