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.

I should’ve explained in my original post. I already have a Beam Gen 1, two Ones, and a Sub gen2. All bought new by me. I want to upgrade for Dolby Atmos and because newer models advertise better dialogue which the Beam gen1 is bad 

Yes, that information would have helped 😂. However, I should have checked your profile.

Go with the Beam2 and upgrade the Ones’ to Era 100’s, if your budget allows. Era 300’s with a Beam2 aren’t the best match they are better used with the Arc Ultra.

2nd Option

• Arc Ultra and Era 100’s. However, the footprint for Era 300’s on stands vs Era 100’s isn’t that huge of a difference. But then again you know your space better than I do.

The dialogue clarity of the Arc Ultra is a vast improvement over the Beam and Beam 2. It is adjustable between off/low/medium/high. My wife loves that we can adjust dialogue needed for harder to hear shows.  I love the added Atmos ambient effects. I would skip over the beam 2 as it has simulated Atmos (which I thought was pretty effective) but no dialogue enhancement setting. 

Thank you for all the advice.  
My current Ones are stand mounted in parallel with my sofa, at ear level. I don’t have room to place them behind the sofa as is recommended. My sofa backs to the LR wall. Watching some movies with this surround setup makes the sound overwhelming. My sister hates watching movies with this setup. Would I get better sound if I mounted the new Era 100s on the wall above the sofa and above ear level?

I really need a floor plan lol

Yes, wall mounting will help relieve the in-your ear effect. Also, you can adjust the volume level of the rears if you can’t wall mount them. However, IMO wall mounting would be best.

I had one’s as surrounds with my Arc Ultra until yesterday. Had planned to upgrade to 300’s, but Costco had a deal on a pair of Era 100’s I could not refuse.  My wife had The same issue with Ones as surrounds that you mentioned. The 100’s have a completely different sound signature and disbursement than the One’s. My wife love’s the difference so far. We tested the setup with Wicked with the Ones as surrounds prior to the change and the sound was very overwhelming from the rear. Then we changed to the 100s and surround is not overwhelming from the rear, but seems more enveloping from the sides and above. She actually, for the first time ever, asked me to crank up the volume for an Atmos movie. 
 

My couch is against the back wall with wall mounts about a foot overhead behind us.