May I suggest the A.E.S. Standard:
Interior dimensions: 10' high x 16' wide x 26' long
You will defeat most potential acoustic problems without special treatments. Absorb reverb to half a second or less, and everybody will hear everything clearly. Record off-the-floor with comfortable set-ups. Etc.
The size should be large enough to accommodate all the amenities you mentioned.
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Sorry to be a PITA, but yet once again: don't even start dreaming without studying this text:
http://www.roletech.net/books/HandbookAcoustics.pdf
Download now and start studying applicable chapters. The alternative is throwing money around and endless half-baked discussions. Sounds harsh, but you will thank the author countless times.
And BTW, I am a "new-comer" in an underpopulated corner of the prairies and yes it took awhile (about a year) to find other available musicians. Retired from the local bar band and just forming a second band with singer-songwriter -- the first in my little garage studio.
Here's my build thread, a modest DIY project, with a budget of about $20,000:
https://guitarscanada.com/index.php?threads/studio-from-scratch-progress-the-final-product.136585/
Check it out for yourself. I am no expert and may very well be one of those guys that has just learned enough to be dangerous.