Advice on dedicated room

I spent a gob of money on a dedicated two-channel room 2 years ago, managing to avoid most of the usual pitfalls.

One of the most successful money pits was Quietrock 545. It REALLY works. 5 layers of drywall, one layer of sheet steel, 1 1/4" thick. Thing about walls is that they flex with the sonic compression wave, turning your drywall into a very low class speaker. Q 545 is a beast that does not flex. Good sound insulation to boot.

Glue and screw construction. None of this nail gun crap. I used two elastomeric products from Chemlink, M1 and BuildSecure. Many cases.

Dimensions also matter, as your intuition tells you. Don’t listen to the usual suspects and their snake oil here; actual scientific research has been done on this subject, and it’s absolutely free. Check out the School of Acoustics at the University of Salford (UK). They did a quarter million simulations to find that MOST rooms are poor, a third are OK, and 2% are good. Half inches matter - I’m serious.

Good luck!

Some diffusion is good, two ceiling fans perhaps?

The air transmits your high fidelity sound. Setting it in motion with fans cannot be a good idea.

Proper Planning Prevents, Piss Poor Performance and don't for get about KISS.
Questions:
1 Save me time or save me money?
2 What is your budget? If answer to 1 is "save me time" forget about cost.
3 What are your performance requirements? This needs to including, HVAC, Power including ground plane and noise (EMI/RFI), Lighting, Acoustically, Human factors, ....
4 Now you have the information to build a model to figure out acoustic, electrical, and thermal performance. Manufacturers post performance characteristics of products you can insert in to models and predict the result. At this point you will need to decide, DIY, Contractor, ... or Engineer.

I did this kind of work as an engineer for awhile where the answer to question 1 was "save me time" and I have seen the work of some others. There are some highly capable people out there in this field of work for every budget. I have also seen some disasters. 

Get some professional help, the result will better and take a pole. "Survey says."

Let's slow down a bit. A lot of conflicting advice. Yes, identify what you are trying to accomplish. No need to over insulate unless you want it quiet for people outside the room.
Once you add drywall, you are adding an element/surface you will need to treat.

Raw ceiling joists are inherently good acoustic surfaces. It may look ugly...but adding drywall...again, is adding a problem you will need to address.

The room is small, yes...but making it even smaller to get 'your golden ratio' may not be the best tactic if you can make it 'feel longer acoustically' through other means.

Your room is somewhat challenging. I would spend a year learning about acoustics and sketching various scenarios out before breaking ground.

Here is a link to a room-mode calculator:

Amroc room mode calculator