Framed like this, I understand your frustration. It's hard to know what's "right" or not without actually experiencing it. It's the bane of the modern way to recording, where we can buy incredible kit for cheap money but we have no way to gather direct experience about how to best deploy it by learning from masters of the craft.
However, consider two things. One, the aforementioned masters often started in exactly the same position. As I wrote above, acoustics is a mix of science (physics) and craft. Heck, physics is, for sufficiently complex tasks (the calibration of detectors for particle accelerators, for example). You can learn the principles and the overall applications, but nobody gets the details entirely right. Even the most engineered recording studios are far from "perfect" acoustically. They just sound "good enough" (where good is "pretty good" ad sometimes "amazingly good"). As with most things, the difference is not between 100% good and something lower but between 20% and 80% good.
And how do you know it's good enough? By the second thing to consider: while you cannot witness the methods first hand, you can easily compare the results. You take your mix, put a limiter and A/B with a good reference. Better still, you ask your girlfriend to listen to both one after the other and gather intelligence on what she thought of the sound. Or a third party. If it's not up to scratch, you ask yourself why.
That's how the masters became so: by relentlessly listening, comparing and correcting to try to get to what they thought was good enough. That's for anything to the acoustics of a recording room to the quality of a mix. Not so much agonizing on the theory but finding out what works and what doesn't. That knowledge can be transmitted and SOS is a good place for that.
In very general terms, from what I've seen (but certainly I have seen very little, and surely the fellows at SOS can find a myriad different examples) a regular apartment room is "small" in studio terms - and a small bedroom is usually a "booth" in a studio. An ok-sized live room is much larger, usually say 60-70 square meters, and has a high ceiling (4,5 meters or more).. it doesn't fit a symphonic orchestra (the "large" rooms do) but definitely fits a dozen players or more. It may have facilities for raising or lowering the "ceiling" (on top of a drum kit for example) and of course the geometry, construction and design may have predictable and specific acoustic properties (such as resonators in the walls, folds and lines in the walls and ceiling, room in a room construction etc). But not necessarily so: after the large "studio 1" and "2" (or "A" and "B") many facilities have far smaller studios - definitely apartment-room-sized which can be used for mixing, referencing and of course the occasional vocal recording.