How do you chose speakers based on room size?

For best imaging and soundstage, you want a tight driver configuration that approximates a point source in smaller room with more a more nearfield listening configuration.

Planars, line sources, etc. with larger sound emitting area can work well in larger rooms listening from farther away.

Also, larger bass drivers in general or possibly a separate sub woofer will be needed to get a fulfilling low end in larger rooms.

Also, many speakers image best with more room to breath around them (distance to walls) so never try to cram too large a speaker into any particular space.

That's the jist of it regarding guidelines I can think of.

Some vendors, like OHM Acoustics, explicitly scale their drivers up and down in size for similar optimal sound in specific room volumes that they specify on their web site..

It costs a lot less to get a great sounding system in a smaller room. Its not that complicated. Smaller speakers + lower power amp typically brings the cost down a lot (except for Magico). And fewer room treatments if you decide those are necesary.

"Duke, That explains alot and makes a good case for a midrange driver that covers as much frequency range as possible. "

Consistent also with my positive experience using OHM Walsh and Triangle 2-way monitor speakers in my small room, both of which cover a wide range through a single driver I believe. Similar expererience using my 2-way Dynaudio Contour line monitors as well.

Ideally, for coherent imaging and sound, you do not want different mid and high frequency ranges to be coming from different locations. Wide range drivers and drivers located closer together physically is what is needed to a greater extent to produce this ideal geometry when listening more nearfield in a smaller room, where the ears are capable of triangulating better on the exact source of sound.

From farther away in a large room, it is less of an issue.

Basically, in smaller rooms, the best things also happen to come quite often in smaller packages.

""In a small room I like a fairly narrow pattern that can be aimed to minimize early sidewall reflections""

For me there are two viable approaches to fitting speakers into a particular room, large or small:

1) Do everything you can to isolate the speakers from room acoustics in order to hear just the recording and not the room

2) accept the room as your particular concert venue and utilize it

In the first case, more directional speakers or external tweaks that accomplish the same thing (see the isolated location of my Triangle monitors in my second system listing) are the solution, particularly in tight or limited quarters.

In the second case, omnis or other wide dispersion designs are the ticket.

Each sounds inherently different but both are viable approaches depending on the listener's goals.

I like and utilize both approaches in different rooms in my house, but, push come to shove, I tend to prefer the wide-dispersion approach.

Audiokinesis,

The smaller OHMs can work surprisingly well just a foot or two out from the rear and side wall in that acoustic dampening material is used inside the can in the wall facing directions to lower output and enable closer placement to walls than would be desirable otherwise. Still I do tend to keep my 100s about 2-3 feet out from the rear wall or so.