How do you chose speakers based on room size?

I haven't seen a guide that discusses how to size speakers based on the room that they will be placed. What is the proper method to mate the two?
Small rooms are the most difficult. You have at least three acoustic issues in a small room:

1. Early onset of reflections (causes coloration and/or compromised imaging);

2. Worse room modal situation than in larger rooms so the bass is lumpier;

3. Often excessive room gain in the bass region (depends on many factors including speaker and listener locations).

In a very large room, the speaker's power response (summed omnidirectional response) tends to dominate the perceived tonal balance, so that matters a lot more than the on-axis response does (unless you listen nearfield), and often in a large or open-floorplan room the boundary reinforcement is insufficient in the bass region so the speaker tends to sound thin and weak. Also the higher power levels required to reach satisfying SPLs in a larger room can tax a speaker both thermally and mechanically.

I recently did a fair amount of experimenting in the course of investigating a small-room-friendly speaker system, and came to some rather ironic conclusions. For example, in order to get the radiation pattern control that gives good performance in a small room by minimizing early reflections, a physically rather large speaker is required.

Now there is a school of thought that calls for very aggressive use of absorption on the walls of a small room, and this relaxes the radiation pattern control requirement. Unfortunately this approach also eliminates beneficial late-arriving reverberant energy.

I recommend spreading out the bass sources as much as is practical regardless of room size as this smooths the in-room bass response (I can explain why if you'd like). Even a little bit of spacing can be beneficial; for instance, a two-way floorstander with the port on the rear down near the floor has the two bass sources (woofer and port) fairly far apart in two dimensions, and this will usually sound smoother than having the woofer and port displaced in only one dimension. If excess boundary reinforcement is an issue, that can often be addressed by reducing the port tuning frequency (which is easier to do than raising the tuning frequency).

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..
As has already been stated, small rooms are problematic. The smaller the room, the more important that acoustic treatments become.

A few rules:

The listening position always needs to be at least 18" from the wall behind. Less than that, and the low frequencies there will smear the sound.

The distance from the speaker front to the listening position needs to be around 9' to give the sound time to integrate. Less space is OK for single driver speakers since the sound is integrated to moment it leaves the driver. This distance really depends upon the distance between the drivers, but 9' is probably safe for most speakers.

Some speakers need to be out from the wall behind. This is especially true of dipoles - planars and electrostatics. These need about 4' from the wall behind. This is so the sound coming off the back will integrate with that coming off the front.

Other speakers seem to be at their best shoved up against the wall. This seems to be the especially case with BLH designs.

Given these parameters, we can see how a small room may become a problem depending upon the speaker. If the speaker needs 4' from the wall behind, and we need 9' for integration and the listening position needs to be 1.5' from the rear wall, we have a total of 14.5'. Obviously, if we do not have that, we may have a problem.

We really cannot do anything about the 18" off the wall behind the listening position. That is a given. The options then become a). selecting speakers that can be placed close the the wall, b) using single driver speakers that require less space for integration or c). a combination of the a) and b).

I am using Klipsch Chorus II in a room that is 10.5 by 12.5'. These are 3 way speakers with a 15" bass driver. This works because these can be placed close to the wall behind. I have space for the 9' of integration and for the listening position to be 18' off the wall. My room has extensive acoustic treatments. Actually, my experience has been that once I had the room under control, whatever I put in there sounded pretty darned good. Before the treatments, it was an echo chamber.

Hope this helps.
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.
Hi Duke,

Have you ever tried DSP as a "room" or perhaps, more appropriately - system treatment?