This review comes from me as a dedicated Sonos User. I personally have complete Sonos Home Theater systems in three (3) rooms. I like the convenience, sound and NO WIRES!
If you're trying to decide between Sonos 3.1 vs 5.1, just look at the sketched diagram in this post.
It shows a complete DD 5.1 setup which include Left/Center/Right speakers across the front, a sub and SURROUND (rear) speakers. In a DD 3.1 setup the surround (rear) speakers are not present. You only have the Left/Center/Right speakers and sub.
As you see, room size is irrelevant.
What you're getting with a DD 5.1 setup is full immersive sound (i.e. Surround) which is not possible with a DD 3.1 setup. The upside to starting with DD 3.1 is that you can expand it to DD 5.1 just by adding the surround (rear) speakers. So in essence it really comes down to your budget.
Remember Dolby Digital is a codec that is capable of carrying multiple audio channels (i.e. left front/center/right front_left rear/right rear and low frequency via a sub).
The only must components are the three (3) front speakers to begin experiencing Dolby Digital. The other components (rears and sub) can be added at anytime to get the full benefit of Dolby Digital audio.
To reiterate Sonos combines the left front, center and right front speakers into one unit which is either a Playbar, Playbase or Beam.
A word on sound quality
Let me tell you something right off the bat.
The sound quality performance you'll end up getting from a speaker will always depend on your room acoustics - particularly room dimensions and speaker positions.
The impact of the combination of these two is actually so strong that in most cases, it doesn't even make sense to utter a single word on sound quality without speaking of them.
This is also why it's not unusual to see completely different reviews of the same speaker.
In one case the speaker might be placed in a sweet spot inside the room and hence the user might be satisfied. In other cases the same speaker might be ill placed and hence user might even have returned it.
The point most people miss here is that it mostly isn't even about the engineering behind the speaker itself. It's about where you place the speaker inside which room.
So, in order to solve this problem, I've partnered with Acoustics and Audio Engineering PhD Andrea Cicero from AC Acustica and created Soundton - a simple, 2D, browser accessible online speaker placement calculator.
With Soundton, now there's a way to figure the sound quality of most speakers before you buy them.
Read more about its working principles at soundton.com/documentation/.
The end colormap provides you the locations with the best (green) and worst (red) acoustics.