room size vs Beam or Arc

My room is similar, 12’x24’. I started with a beam and ones as surrounds in that space, and it sounded great. Added the sub shortly after and that took it to the next level.  This past year, I upgraded to the Arc Ultra, and moved the Beam to the bedroom.  The Arc ultra was a huge upgrade to me, although others may not agree. I have found all the setups adequate for the space, but have enjoyed the upgrades over time. I don’t think any soundbar alone will blow you away. Adding either the sub or surrounds is where the magic starts to happen. My next upgrade will be changing out the ones for era 300s as surrounds.

If you don’t plan to upgrade over time, I would probably start with arc ultra and surrounds. 
 

If budget allows go with the Arc Ultra if not follow the upgrade plan suggested by ​@wds0001. Definitely go with the Era 100’s. The One’s would be used and/or refurbished and the tech is lagging behind all current Sonos gear and into the future. Again, If budget prohibits  the Sub 4 then consider the Sub Mini.

A budget compromise for a complete Home Theater is:

• Beam2
• Era 100’s (surrounds)
• Sub-Mini

Note:

A sub does more than just provide good low-end. It also relieves the soundbar of low-end duties and allows it to better handle the mid-range, vocals and highs.

A lot comes down to your ears and your expectations for many a Beam alone would be enough in that room, as a big improvement over the TV speakers. The Arc Ultra will add a lot more, better Atmos, better low end and better imaging from the greater speaker spacing.

A Sub or mini-Sub would be another big step up, which one again depends on you, the mini is going to be a nice improvement but the full Sub is going to add more impact.

The 100s are the minimum to consider for surrounds, they sound pretty good. I use a pair here, tried 300s and while the sound was fantastic placing them to sound their best and keep the spouse happy wasn't possible, hence the 100s solution.