What are advantages going to 7.1?

More fill in a larger room and more accurate reproduction of actual 7.1 media.

At what point does a "larger room" start?

I would say that if you are doing renovations or building a house or dedicated HT/media space go for the 7.1 if not since there is so little actual 7.1 content 5.1 will do you very nicely. I went 7.1 but I was doin major renovations and figured while I am pullin wires why not 2 more.

I have a 7.1 system in a 15' x 25' x 8' area. I have a good 8-10' behind my listening area where my two rear surrounds are located. If you have no room behind your listening area a 5.1 system would work better. You just need "area" behind you to validate having a 7 speaker system.

At what point does a "larger room" start?

I'll clarify:

As long as you aren't sitting up against the back wall, and your back wall is at least 15' from your TV, you'd probably qualify.

Also browninggold makes a good point, you need room behind your seating position.

You can never have enough speakers.

What are advantages going to 7.1?With proper set-up (seating away from the back wall), there are three noticeable advantages: better wrap-around envelopment (4 speakers can literally 'surround' you better than 2 speakers can), clearer rear-vs-side directionality (4 hard sources instead of 2), and greater imaging stability (no matter where you're sitting, sounds from your sides and behind you stay locked in those directions, because there are speakers there).

How far would I need to be from the back wall? What's an acceptable minimum?

I have the same question. I am currently running 6.1 with a rear center, but was thinking of going 7.1 as i am finishing my basement. I have about 10' behind the seating area. Is this too much? should i stick with the 6.1 and mount the rear center about the seating area?

Thats what I have is 10' behind me....IMO I would think 3-5 feet behind the rear seating area would be adequate.

How to choose these two side speakers? Should they match rears?

How to choose these two side speakers? Should they match rears?

They should match. If they can't and you have to make a choice, put the better of the two pairs as the side surrounds.

How far would I need to be from the back wall? What's an acceptable minimum?I'll second the recommendation for 3-5 feet. You just want to make sure that there's some meaningful separation between sounds from your sides and sounds from behind you. I have about 10' behind the seating area. Is this too much?No, that's wonderful. Remember to spread the rear speakers at least 12 feet apart, so you can hear left-vs-right stereo separation behind you.