What's ideal size for a listening room?

Largest dimension of the room can determine the accurate reproduction of the lowest frequency of your audio system. At the sea-level where speed of sound is 1,130 ft/sec, longest dimension of the room should be about 56.5/2 = 28 ft to capture one half of the 20 Hz audio signal. This calculation is based on formula v = fxw where v = velocity of sound, f = frequency, and w = wavelength. 18 ft long room can reproduce 30 Hz and 14 ft one can reproduce 40 Hz accurately.  Your speaker specifications (3dB cutoff) can be used for this calculation. This is not a requirement for subwoofers as they reproduce lower frequencies by bouncing the sound off walls many times. Above theory applies only if you want accurate reproduction of the sound.

Show-boxed shaped rooms are subjected to many room modes where you will hear certain frequencies amplified at certain locations and dead spots on other locations. A trapezoidal shaped room can minimize most of these room modes. If you have room where roof/ceiling is at an angle, then it will most likely be a trapezoidal shape or one closer to it. I understand this may not be an option for some, but at least in theory those room shapes are better than shoe-boxed shape ones. 

This is just a start. Next you have room treatment which is a very big topic covered very well on this site. Do a search.

@pwerahera 

Thank you for the really Great reply and gives something to think about related to lower frequency waves in relation to room size.

I've done dirac sound analysis of the room and find that all frequencies seem to be coming through quite well and then when enhanced with DSP for home theater it makes a huge difference, really smooths out the curve.

Thanks

The bigger is the room, the better, just as simple as that. 

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!