This is a huge topic,as most here know. I am facing similar decisions coming up because I too am building new rooms. I have 9' ceilings and I wish they were higher. What I have decided to do about width and length is make practical decisions on "size" based on comfort in the control room balanced against having enough tracking space for one large room and two iso rooms and a couple of amp iso booths. It occurs to me like this. Over time,different people have done very deep studies of mode problems in typical rooms. There are dimensions that are more ideal than others for sure but,I am ,for the most part unable to achieve these so, as long as the modal problems can be calculated and known to me, I am going to build so that the spaces make sense regarding the day to day work flow and so on. I think the key is being able to properly treat the rooms so I can have the best combination of good sounding rooms and "practical" spaces. There will be problems no matter what the dimensions are. Treating above 200 hz seems to be relatively "simple" compared to 200 hz and down. The early reflection issues can be dealt with using 703 built into frames and properly placed on walls and ceilings. I am now reading a great deal about how to properly deal with the standing waves in the bottom end. There is a lot to consider here to say the least. Mode spacing and amplitude. Which method to use? Tube traps? Panel absorbers? and a whole bunch of other stuff. I find that problems in the bottom end are the hardest to solve and if one really takes the time, they can make a huge difference in the way a given room sounds. One mistake I made in my last two rooms was over treating the high and mid end. Thankfully,I have more space this time around and I hope to have a more open sounding space while at the same time,dealing with reflections that can distort what goes into my ears,and mic's. This is harder than it first appears but,again, it is a doable thing if one is willing to dive in and read read read..