When is a Listening Room Too Big

I've always considered the interaction of your choosen speaker and the size/type of listening room to be step one in getting the best sound possible. A speaker too big for your room will overload everything and ruin the sound, a small speaker in a really large room might only work well with nearfield listening.

Here's my question; when does a listening room become too large? Lets say you have a nice speaker like a Magnepan 20.7; my current room is 17.5 w x 26 L x 9 h. As I design and build my next dedicated listening room, what dimensions should I aim for? Is 21 w x 31 L x 10 h too big?

Paul Klipsch always said that the best measured rooms typically fall in a range where the width is around 67% of the length...
Some of the very best sound I've heard involved large systems playing in very large rooms. One setup was a dedicated listening room that was about 27' x 50' x 12' or so (just guessing) with a massive horn-based system. The other was a system using three Wilson MAXX 2s and two WP-7 in a pretty large conference room (at CES). I have also been impressed by a quite small system (YG Carmel) playing in a quite large ballroom. The space helps very much in giving the presentation proper scale. Wide spacing of the speakers also makes it possible for more than one person to be located in a sweet spot (in almost all typical home setups, the sweet spot is really far less than one foot wide).

With something like a Maggie 20.7, there might be some restriction on size if one wants to listen at high volume because of the amount of power that would be needed and the possible limitations on the speaker delivering high level of bass. But, I bet that at reasonable levels, Maggies would sound great in a large room (I have only heard them in a room of about 16' x 25'). Just a guess, but your hypothetical big room would not be too large and would be great for delivering realistic size to the soundstage.
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.
You are right about sometimes less being more, especially in the case of a very lively larger room.

I think large rooms work well with Maggie in particular. The best Maggie setup I ever heard (and among the best I have heard anywhere) was at Jim SMith's Audition shop in Birmingham AL. SHowroom was large, but not huge. The panels were positioned almost in the middle of the room with lots of space behind and slightly more in front. I was blown away and bought a pair right then and there and had them for years until I moved into my current home where my room is fairly large but L shaped and narrow and optimal placement was a problem.

A sub or two or four can always be added if needed, but I suspect the other charms possible in a large room has a good chance to float the boat alone.

I tend to think of most peoples rooms as the main bottleneck with MAgnepan speakers. Most are not large enough to really let them be set up optimally and shine.

Yes you will likely want a good bit of power and maybe even a fairly beefy tube amp to drive them to higher volumes in most larger rooms though, if that is something that matters.
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.

When is a Listening Room Too Big
Buy bigger speakers?
why did you not look at the info provided by George Cardas on this subject? it's been on his website for a really long time. Or, should I assume you've already read this & still have questions??
here's a link: http://www.cardas.com/room_setup_faq_page1.php

there's a lot more info the Cardas website which should interest you.....
I read that long ago probably; will check it out

The best sound for me happens when a speaker " loads the room " with sound. The bigger the room the more air must move for that to happen. I not a big fan of large speakers,, ---big speakers big problems---
Um, sell the house and give some money to charity?!
Or but some big Tannoys or better yet Kinoshita RM-7V!
"Rooms with open floor plans and cathedral ceilings tend to less than optimal for serious listening."

That describes my listening room exactly and I agree that this type of room can be difficult. The cathedral ceilings seem to reflect a lot of sound right back at my listening position in roughly the middle of the room. This tends to muddy up the sound and the imaging. The bass is also rather uneven, with large dips and valleys over the bass frequency range. I have had to add a lot of acoustic treatments around the room (bass traps, acoustic tiles, large sound-absorbent furniture, etc.), which does help a lot. Also I have had to work a lot on speaker and sub placement to minimize the bad effects. It's pretty good now but it took a lot of work to get there and it could still be better.