How do you chose speakers based on room size?

Different speaker types (dynamic/cones, panels/line sources, and electrostatics, all have different set up/room size requirements. Generally in a smallish room, for optimal set up, panels, line sources, and electrostatics will not work as well as dynamic ones and these should be of the stand mount type which reduces the distance required from speaker to ear needed to get proper integration of the multiple drivers.

You really need to feed specifics re room size, listening preferences, and set up restrictions, to get any kind of a meaningful answer. There is no meaningful 'guide' but you can get some good practical advise if you provide some more info.

An oversimplification would be:
Small room - small speaker
medium sized room - medium sized speakers
large room - large speakers

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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).

Duke
dealer/manufacturer

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..