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?

@rixthetrick 

The maths is noted, but unfortunately only an approximation of the true equation so ,fortunately for me, not much help.

The problem with using a small room in 'landscape' orientation is that on the short dimension there isn't space behind the speakers and behind the listening position.  So if the speakers are moved forward to give them air from the front wall, as you say, the listener hears bad reflections from the rear wall just behind him.  It isn't easy to damp those.  It may well be better to accept some side wall reflections adjacent to the speakers.

Yes you have it - it's all a compromise.