5.1 to 7.1

Actually, I think room size is an issue. If you are going to put the 7.1 speakers within a foot or two of your listening space then they can actually detract from the sound. If you have multiple feet behind the listening area, they can add. But too close and they just become point sources. Some people put the back speakers on the floor behind the couch/chairs and pointing upward. I tried that once and found it quite unsatisfactory. If you have enough room, they can really fill in the sound. But, IMO, you need space behind to make them really worth will.

FYI - I have a 7.1 theater with plenty of space and the back speakers really add. I have 7.1 in the family room with only a few feet behind the chairs and the back speakers are interesting but honestly do not add that much. They are too localized.

If you have some extra speakers of any kind, just run some wire across the floor and give them a try. It will at least give you an idea of the look and the sound.

Think of the same triangle rule you use for your fronts. I would suggest 5 feet back, which is hard for most people. You can also consider 6.1 which takes the stereo issue out of the picture. You can also use 6.1 with 2 speakers in parallel.