Resonances are calculated by 1130/2L
So 11’ ceilings = 1st order node of 51.36hz.
Ideal room is 10h x 17w x 23l
Avoid rooms with common dimensions or squares.
The neutral points for modes are 0.2/0.32/0.45/0.55/0.68/0.8 times any dimension. Putting listeners on these ratios, or speakers/subs ensures more neutral response. Avoid sitting in the middle of the length dimension. -Null at room fundamental and peak at 2nd harmonic.
Increasing height increases volume. Practically, there is no reason to expand height unless you are looking to accomodate tiered seating or as Rich is, a large screen. Beyond 10’ you are causing issues unless you plan on treatment. Long bare reflections = long decay times = Not nice. Long decay times lead to poor dynamics and decreased speech intelligibility (especially on the length axis.). Expect unpredictable zings and boings in tall ceiling rooms. Pop a balloon and see.
Large rooms trend to longer decay times in general. In reproduction, we want shortish times. In live sound we want longer times. In practice, it totally depends what you are using the room for and how many listeners, etc you are looking to fit. I would recommend you buy a reference book and stop online research. Lot of boneheads on AVS and youtube that will end up costing you wasted time.
FWIW, you can make any existing room feel bigger with diffusers.
