When is a Listening Room Too Big

My advise is don't tailor the space specifically to your Maggie's, unless you are sure you are going to die with them. I would look for general dimensions that would work well with most medium to large speakers in case you have to upgrade from your Maggies someday.

I wonder about a larger room just for that reason; scale. It's harder to convince yourself that Allison Krauss and Union Station are standing right in front of you when your room is 'small'. I think that all the Maggies are capable of amazing realism (especially the 20.7's) but if the room is too small it doesn't truly work to its total potential. If I had a smaller room I would get 3.7's...or a different speaker.

I may actually have the opportunity to build a new room once I retire (not that far away!) and it will most likely be the last one for me... I'm thinking that another 3 to 5 feet, say up to 22 ft or so wouldn't hurt.

The amps I use now are plenty of horse power for my current room (Cary 500 mb that double to 1,000 watts into the 4ohm load) but maybe they would not be up to task if the room got significantly larger... Wonder if there is a 'mathematical' way or rule-of-thumb for estimating power currently used vs what you would need when the room gets larger?

I've built large med small rooms for audio use the large is fun with giant horns but at times the size makes it less cozy and intimate. Can be a bit harder to relax in a large space. My largest is about size you list but with taller sloped ceilings If its just you in a big room with a system doesn't seem as natural to me I use my medium space more often. The large about 2 twice a week. YMMV.

Good point by John regarding how one may have a preference for a certain size room that has nothing to do with sound quality.

I have 3 rooms running speakers off my main rig using in-wall speaker wires that run from the room where my gear is located. That room is 12X12 with standard height ceiling.

Above it is the sunroom, similar size, but cathedral ceiling.

Then I have my "large room" which is ~ 20X30 but L shaped, so smaller in volume.

I listen a lot in both the main 12X12 room and the larger L shaped room. Each has their unique charms in terms of how the music is presented, like going to different size venues for a live performance. Which is best? Hard to say. Smaller room is cozy and intimate and sounds great. Good for getting intimately involved with the music. Larger room presents things on a larger scale and is good for being able to just sit back a bit and soak it all in and also sounds great. Hard to choose just one, but I could live with either if I had to.

Sunroom has its own appeal and belongs to my wife, so my options are limited there and the sound is quite good there as well, but I use it more for casual rather than critical listening.

Rooms with open floor plans and cathedral ceilings tend to less than optimal for serious listening.

Shakey