What's ideal size for a listening room?

The Rooms can also be evaluated on the Amount of Apertures and Aperture Dimensions.

Doors and Windows on External Walls and Internal Walls will have a strong influence on the balance of End Sound.

A Hollow Door used on the Room Entrance will most likely create a Reflected Colouration to the Sound being Produced.

It has been known for a inner panel to be produced and attached to the Internal Face of the door, where a Cavity of approx' 20mm is filled with a highly compressed fibre insulation or Koln Dried Sand. When a door is given this type of treatment there are usually additional seals used to prevent air movement between door and Door Fame and Door and Threshold.

Depending on age of Widows Air Movement might need to be controlled with a Clear Tape typically used for Winter Periods as an Air Seal on Leaky Windows.

Controlling reflections of the Glass is more complicated, but a Heavily Lined Curtain is the cheap option with limited effect. Padding the Curtain with a Fibre will help furthering the curtains function for controlling reflective sound.

https://perfectacoustic.co.uk/product-category/acoustic-curtains/

The Room is the Room, the Room will be much better as a Space to manage Sound being produced, when measures are adopted, to make sure the End Sound created by the Audio System is only heard once by the listener.

When the End Sound produced within a particular space is coming from Speakers that are adequately coupled to the space, the notion of being in a particular sized room of a particular ratio will quickly move on.

The Bliss being experienced, the sense that Sound is not adulterated and not coloured is to Supersede all other notions.   

@emergingsoul 

The larger room on your list can still be filled with modest hardware- it is not so large as to need enormous amps and speakers.  My current room is a little larger- 17 x 27 with 8-ft ceiling and I can be happy with a tube amp and KEF LS50s.  

The larger room will give you more flexibility in choosing speakers and making sure your speakers and listening spot have plenty of room to breathe.  Neither like to be close to wall boundaries! 

You can use many site calculators to predict what modes you will have in a room, or likewise, try to design a room that avoids piling up of room modes.. For instance, AMROC has a calculator where the results are visual and as you slide your cursor up the frequency chart it shows you where in the room these heavy concentrations are located. They also have a 3D version that lets you draw your room to your exact size if you have a sloped ceiling, like I do. Although, the results were not much different than the 2D version w/o the slope. Second decimal place deltas.

There is also a decent room mode tool in REW that nicely shows you where the tangential, axial, etc. line up or are nicely separated.

I thought I would be golden with the room ratios of 1 : 1.65 : 2.33, but I still have excessive bass below 60 hz and a series of bumps along the frequency response. And I have some room modes stacked up due to nothing other than the room dimensions that cannot be changed.

https://amcoustics.com/tools/amroc

https://www.bobgolds.com/Mode/RoomModes.htm

I've treated where I could and have "fixed" most of the bass issues. Check out my 2023 TN Systems page for some REW outputs.

I'd go with the larger room, volume can help cure many ills. Do plan on treating the room with absorption and deflection. GIK Acoustics gives free consultations and has an acoustic calculator as well. A good sized wool area rug can be very helpful (not a synthetic rug as they are actually reflective) especially with an all natural pad......do not get one with synthetic backing, which is also reflective

@kofibaffour +1 LOL .. at 6’ deep? 

"A room with these ratios is ideal:Height - 1m Width - 1.618 Length - 2.618m"