Small drivers vs big drivers

@quanghuy147

To make the situation more complicated, some speakers have small drivers but there are many of them. Can many small drivers compensate for the size limitation

Yes, they can, but it's a whole different design ball-game! Generally, smaller diaphragmes are lightweitgh & therefore move faster but, being smaller, they reproduce lower frewuencies at an imperceptibly low amplitude (volume).

I don't know which specs determine a wide sound stage

The wide s-s is usually the result of efficient driving (i.e. small speakers) and a small baffle (small-sized speakers).
Numerous other design parametres are also invlovled, but let's not complicate the issue!

For difficulties in reproducing low frequencies through smaller drivers, see Shadorne above.

There is not a 9” on the planet that can go down to 18Hz, so don’t know where you got that.

Besides just driver and cabinet size, driver xmax is important, that’s literally how much movement the driver can do, the more it can move back and forth, the deeper it can go.  
  
Soundstage is the off-axis of the speaker. If Speaker A has more output than Speaker B at 90° (perpendicular), than the soundstage will be wider. In terms of the quality of the soundstage (aka imaging), that’s how the off-axis performs, it should be very close to the on-axis.  
 
This speaker will have a wider soundstage and better imaging than this speaker.

Despite multiples of 7" or 8" drivers having the same radiating surface when calculated mathematically, there's something about a 12" or larger that just sounds better.  Designers went to smaller drivers to make speakers narrow, for ergonomic and stylistic reasons; there's something really special though about a wide baffle speaker that can accommodate a big woofer on the front.

Adding to @twoleftears :

The 2.5 way speaker is a little miracle of efficiency, space and capability.

Two 8" drivers are still only the radiating surface of a 10" or so, but the increased efficiency, and reasonably extended bass output make them superbly enjoyable compromises.

Best,
E

@twoleftears

+1. A 12” or larger woofer can pressure up the room much like a real kick drum would do. The difference is very noticeable. The tendency of speakers with smaller woofers is to generate the lowest frequencies through port tuning which gives a muddy sounding kick drum, as the transient response is smeared by group delay. The frequency response on small multiple woofer speakers can be sometimes equaled to that of a large woofer by clever use of ports to plumb the subsonic depths but the time smearing from porting leaves one with the impression of a kick drum that sounds a bit like a bass guitar - it hums or resonates rather than sounding explosively punchy.

Large woofers really make a kick drum sound realistic and much more distinctive from bass guitar despite sharing similar frequencies.