As for your "telepost" concern (not sure what that is, ...maybe a support beam?), you'll have to decide that, and work that out.
As for room dimmensions, I strongly suggest using what world renound HT designer Russ Herschelmann uses in his theaters(designed Disney worlds IMAX theater, writes for HT mag, SGTHT mag, and AVinteriors on theater design, acoustics, engineering, tweaking, etc), He uses a modified 6% Bonnelo criterion. What he does is makes sure that room dimmensions are within 6% ratios of each other.(maybe 6-9% at most). What this does is garantee that room modes end up not doubling up or stacking up on each other, compounding problems, and allows for smoother bass modes in the room, rather than more pronounced ones. What you want to avoid(since you will have room modes no matter what) is double or tripple room modes that are stacked ontop of each other, which often happens with rooms that have boundaries and reasonant frequencies that are multiples of each other. So, in your room, Russ would take your fixed 7'4" ceiling, and then figure the width by doubling your width 7'4" x 2, which is 14'8", then add OR SUBTRACT 6% to that width (which is 15'6" or 13'9"). Then, for the length, you would multiply 7'4"(or 88") x's 3, which is 22', then add or subtract 6% or so(for a length of 23'4" or 20'8" or thereabouts!). What happens is, if you figure the room modes for these dimmensions, you'll notice they don't stack or "double up" on each other! This is what you want.
Anyway, you can figure room modes by looking it up on the net. Good luck, hope that helps.
As for room dimmensions, I strongly suggest using what world renound HT designer Russ Herschelmann uses in his theaters(designed Disney worlds IMAX theater, writes for HT mag, SGTHT mag, and AVinteriors on theater design, acoustics, engineering, tweaking, etc), He uses a modified 6% Bonnelo criterion. What he does is makes sure that room dimmensions are within 6% ratios of each other.(maybe 6-9% at most). What this does is garantee that room modes end up not doubling up or stacking up on each other, compounding problems, and allows for smoother bass modes in the room, rather than more pronounced ones. What you want to avoid(since you will have room modes no matter what) is double or tripple room modes that are stacked ontop of each other, which often happens with rooms that have boundaries and reasonant frequencies that are multiples of each other. So, in your room, Russ would take your fixed 7'4" ceiling, and then figure the width by doubling your width 7'4" x 2, which is 14'8", then add OR SUBTRACT 6% to that width (which is 15'6" or 13'9"). Then, for the length, you would multiply 7'4"(or 88") x's 3, which is 22', then add or subtract 6% or so(for a length of 23'4" or 20'8" or thereabouts!). What happens is, if you figure the room modes for these dimmensions, you'll notice they don't stack or "double up" on each other! This is what you want.
Anyway, you can figure room modes by looking it up on the net. Good luck, hope that helps.