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.