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.

Saki, Speaker placement (including the rooms that they might work well in) is critical for just about every speaker design.

Some folks pursue set up diligently until they have optimized it, some more than others until their significant other or their own sense of asthetics takes over, some not at all and everything in between.

Your suggestion raises the toughest and most common (I think) situation facing the majority of audiophiles. Good sound in a small room. Tough under the best of circumstances and especially so if your expectations are demanding, but to exclude placement as a consideration presumes that there is a speaker with audiophile pretensions which will sound good no matter where you put it or where you sit when you listen to it.

I don't know of such a speaker, let alone provide 'simple guidelines' or advise, apart from getting a pair of sealed enclosure cone driver monitors that can be placed near a wall or on a book shelf, a sub woofer to fill out the bottom end, and an equalizer of some sort to balance out the very uneven sound that would be likely. Not very good advise IMHO unless you don't care about things like a balanced FR at the listening position as well as a full (wide/high/deep) sound-stage.

FWIW.