is too large a room no good?

I used ATC SCM 20 and a sub in a room with space for a complete badminton court, hot tub and large ornamental fig tree. Approx dimensions were 50 feet by 36 feet with 11 to 18 feet vaulted ceiling. No problem at all to fill this space (but I absolutely needed the sub) and I like to listen extremely loud - everything was wood with windows along two sides...not the best of acoustics due to the reflective gymn floor.

So if you get speakers with very wide and even dispersion that play at pro SPL levels with low distortion then they will fill any space well (won't fix reverberation issues but will fill the space evenly).

Without naming names (and upsetting owners), my experience with these small ATC bookshelf speakers compared to several other brands of consumer speakers (big well known names) caused me to radically change my opinion about non pro designs with non-pro drivers. Today I would not use anything but a pro design in a large space.

The Goldilocks room?

Very small rooms = bad. Period. There is no such thing as a great sounding very small room. Good or Bad is possible with "small" rooms but never great. Too many close reverberations give a highly cluttered sound and listening in a near field position (less than 3 feet from speakers is only a wee bit better than headphones). Besides finding a sitting position away from a wall becomes a challenge. A diagonal speaker set up with the listener closer to the center of the room (but obviously not precisely at the center) usually works best in a very small room (get the listener and reflected speaker sound as far away or delayed in time from walls as possible). This is the same reason why open headphones always sound better than headphones with acoustic isolation (cups).

Large domestic rooms/spaces can be great to good but are very rarely bad (again a lot depending on dimensions and reflectivity of the surfaces will determine how great it is) Up to 50 foot dimensions seem to work well. Then long delayed echoes can start to be a headache.

Giant rooms can sound good to awful - again depending on the space. For example, a low ceiling in a large space sounds terrible and highly reflective surface in a very large space can lead to deeply disturbing long time coherent reverberations from any large flat surfaces - such as an auditorium, gymnasium, or double story windows in a giant North American style open living room (as much as possible sound should be directed into the room away from the windows and listening position well away from windows in this case).

A small room being better than a large room is extremely rare. This is because sound travels and loses energy with square of distance(6 db drop in intensity with each doubling of distance). Furthermore a better separation in time will help inteligibility of sound.

The Goldilocks room is actually not too big and not to small - it is just right - but then you probably guessed that! FWIW - Thick Carpet is as near as dammit the best thing since sliced bread when it comes to any room!!