How to make small room sound bigger

Quite a number of posts here are not answering OP's question, which I must say I find a strange one.

How to make a small room sound bigger.

Why?  My objective would be to improve the SQ.  As some have posted, making the room sound better will not necessarily do that.  In a concrete shell the sound will sound like it's in a big room because of the multiple reflections with extended overhand period.  Agreed it certainly won't sound good, but it will sound big.

Room treatments such as those sensibly advocated above will certainly improve the SQ but they won't make the room sound bigger.

OP, perhaps you will explain your objective more fully?

Having a small room myself, one has to accept that there are limitations to how "big" the room can sound. And as has been observed above, lots of reflections will make the room sound bigger but not better.

Really, as with any room, a judicious use of absorption and diffusion is what's needed. I would start with the back wall because, properly treated, it will allow you to get further away from the speakers, which in turn gives a bit more flexibility in terms of placing the speakers vis a vis the front wall.

Sidewall treatment is helpful but can be difficult in a small room as more likely than not one of those walls probably has a door which may be placed at a reflection point. Treatable - but with a serious compromise in regard to aesthetics.

Keeping the speakers a reasonable distance away from the sidewalls also helps.

Perhaps the most difficult challenge in a small room is the tradeoffs between bass extension and soundstaging. It's difficult to get maximise both in a small room. The amplifier/speaker interface is important in the former regard.

Funnily enough, quadratic diffusion for me anyway, seemed to open up my room sonically.
So I’ve read, in a small room where the boundaries will not allow for quadratic diffusion angles to fully develop, binary amplitude diffusion (BAD) with absorption is a winner. On quadratic diffusion there is a minimum distance where the diffusion angles have opened up the different frequencies, there’s charts and graphs and such available.

GIK for example have diffusion / absorption panels, acting in a similar manner to BAD panels, which can be totally DIY with plans available online (BAD plans online, not GIK plans).
This may be the way I go with my ceiling at first reflection point, and I have also seen a BAD with a curve in it, adding further more diffusion, but also in a reasonably predictable way. A 2D pattern is created using primary numbers for the holes in a BAD.

It may not necessarily make the room sound significantly or exactly like a larger room, but it can make it react not like the small room it is by removing flutter echo and the likes. In a larger room reflections bounce across the room and interact with the sound energies, this is artificially achieved through quadratic diffusion, the frequencies are cut up and fanned out, developing in a smaller space that which a larger room does. It’s the lower frequencies where this simply cannot happen with diffusion, there’s no getting around this, that I know of? But then, aren't lower frequencies considered non directional? However the long waveforms (low frequency) simply don't fully develop in a small room, there's not enough distance.

I have just built six folded well diffusers, and I have to temporarily install them to be able to tell what they’re going to do in my room. As space is a real issue for many of us, the shallow depth of the folded well diffuser, combined with the shorter working distance of the devices made it an easy choice for trial in my room.

At four inches deep, with a fully developed working distance of 5 1/2 feet, this could work behind the speakers on the front wall without taking up too much real estate.
And without compromising a suitable listening position, there is a reasonably broad frequency range (mine are 40mm = 307 - 7503Hz) they work at.
Plans: https://dngmns.home.xs4all.nl/fwd_uk.html

These are all straight cuts, all DIY, and I’m not handyman genius I assure you, anyone in here can make these.

I’ve decided to get diffusion done first, measurements, then absorption based upon measurements. After Mike Levigne’s music session, I was sold on diffusion, which is all he has - it’s incredible, really.

With my limited room, I’m running cables under the big 17" deep QRD17 diffusers, but on the sides laid up against the wall, those are the folded well diffusers finished.

If you’re on a budget, and have any (I do mean any) handy man skills these are pretty damned easy to build, there’s Youtube videos as well. However, again I will say in a small listening room the back wall, representing the most first reflection energies coming off speakers, I’d certainly suggest BAD diffusion with absorption built in.

For free information:

@ei001h - there is a benefit of a smaller room that aught to be mentioned, that is cabin gain. If you take care of flutter echo, low frequency absorption and room nodes and the usual suspects, there is less energies required to pressurize a smaller room.
Here is where the benefits of a stand mount speaker with smaller baffle, smaller cabinet sizes (potentially less cabinet resonances) come into it's own, and the cabin gain loads up the room on lower frequencies.
A small room can sound very good too, done right.

The small room requires room treatments to sound good, which has been mentioned above. In a small room, listening in the near-field is basically a requirement. This will allow for the room to be less of a factor. Also, IME no small room can actually portray ’scale’ like a larger room/larger speaker. Small room also requires, again IME, small speakers...generally stand mount to sound best. A sub woofer, so long as it is a smaller sub woofer, can be made to really boost the lower frequencies. BTW, I don’t consider a room with 14’ on one wall that small....to me walls that are 8-10’ apart are more typical of small rooms, albeit rooms with low ceilings are more challenging. Therefore, I also think the volume of the room has a lot to do with potential SQ, not just the distance between walls.  As others have stated, a small room can sound great, but it needs a little thought and preferably it needs to be dedicated to the system.