@james1911 I have a dedicated listening space within my home, and have also spent much time investigating a design for an outside of the home listening and entertainment space.
In relation to building a purpose designed space there is ratio calculations to be found that prove optimised for managing sound within a space. If your dimension selected is at a place where a dimension tweak is achievable, I'm sure there is a ratio that will be almost ideal to fit in closely with your selected dimension.
Room Size is a consideration, but also is the Room Structure.
The Floor is very important, with in my view a concrete slab being the least complicated where a Interface with a Audio system is being considered. If this is a longterm space being created, it is also worthwhile considering a extra deep concrete is used, which will be more effective as a result of the addition of mass.
Wall Structures are in general able to be improved for being rigid and for adding mass to.
Brick or Block with an insulation on the outside fascia and overclad, will leave a good blank canvas for the interior. An insulation on the internal walls will most likely be overclad with a drywall board, which will present flexion issues when exposed to transfered amplitude from sound produced. This can then manifest as new produced sound within the space, (not wanted). An internal brick skin is more desirable and is aesthetically attractive as well.
Timber Frame Walls are typically designed for minimum framing for acceptable structural properties. When it comes to Audio Purposes, the design for a Timber Frame Wall is able to be improved. Increased Framing, i.e, increased Verticals and increased noggings. A Plywood Stretch Skin Internally prior to a Drywall Board. Insulation types for cavities are known for their acoustic control properties, by the - ?? Kg/m3.
The Roof / Ceiling is probably best when not over complex and Kept as a Typical structure. Interior Room Treatments will be plenty to capture and control transfered amplitude from produced sound.
As advised reading up on the design options will be very beneficial, and the Space will substantially benefit from room diffusion / absorption treatments being added.
I strongly suggest, putting the focus on the Room, and forgetting Speakers for a while. As the space Speakers are to be used in, if not optimised for sending sound. Might bring about an assessment formed on an experience being had in a space not prepared as near optimised.
Current owned Speakers, (being a transducer) can be used to assist with the learning about how Electrical energy being covered to Kinetic energy (sound) is delivered as an amplitude and energies present are being managed in the new space.
