There isn’t going to be any “perfect” one-size-fits-all room for all situations. For example do you plan on one or more rows of seats? How many seats across? Why restrict yourself to a rectangle? How many speakers, (wide and multiple surrounds can accommodate longer rooms better)?
With only one row of seats placing the screen on the long wall is a viable option since a wider room allows more flexibility in placing the mains and with greater standoff of the surrounds you can get a more even surround sound field over you entire seating area than if the surrounds are very close to the side seats.
Based on the rooms I’ve used so far I would make the length and width such that the standoff distance to the nearest seat from a side or rear wall is proportionally greater the more rows/columns of seats I have. So say for one row of 4 seats across I would want a minimum of 5 feet to the sided and back walls from the closest seat. I would add a few feet in each direction for every row/column of additional seats. This creates a more even surround field for all the seats rather than just the sweet spot.
I would also want to be able to place the mains outside the farthest L/R seats, also proportionally farther the more columns there are, while always keeping at least a 3-4 foot standoff from the speaker to the wall to allow some flexibility in placement to create the best front soundstage.
Also, just because you don’t want length walls to be equal, doesn’t mean that a square listening area doesn’t work great. The most seamless 360 degree soundstage I’ve had was in my apartment where the room was 16 feet long by 12 feet wide. By building a curtain wall in the front I turned the listening area into a 12x12 which allowed me to place my speakers in a perfect circle around my seating. This created a perfectly seamless soundstage with only 5.1 while maintaining the advantage of a rectangular rather than square area for standing waves.
Additionally by having more distance to the side walls while also sitting closer to the screen it greatly cut down on reflected light from the screen bouncing off the walls (both my apartment and present home theater had/have black curtains, floor and ceiling). But even with total blackout some light reflects off the side walls making it harder to suspend disbelief in my present home theater than in my apartment where the screen just seemed to hang in empty blackness. So don’t assume that only acoustics are affected by room size/shape.
