The only reason I mentioned room size vs seating location is because I didn't want the original poster to automatically assume that his room was too small for a 7.1 set-up. Besides, the point of adding surround-back channels is to have rear vs side localization in the surround field and to fill the sonic hole behind listeners that occurs with traditional side placement of the surrounds.

As I mentioned earlier, I've seen large rooms that were poor candidates for 7.1 because the couch was against the back wall. Likewise, I've seen small rooms that were able to pull of 7.1 very well: a friend of mine converted a spare bedroom into a recording studio. All 7 speakers were a little further than arms length away (TDL nearfield monitors). Since they were nowhere near the room boundries, the imaging was superb.

So, irrespective of room size, seating location is the important factor in determining whether a room is a good candidate for 7.1 or not.

As for distance between the two pairs of surround speakers; it's a balancing act. You want side vs rear separation for distinct localization when watching movies; but you also want side and rear blending for better envelopment when listening to music. Requires listening and experimentation, but the results are worth it.

Best,
Sanjay