My room dimensions are nearly identical to yours and I use an even bigger screen: a 100"x56" Greyhawk. If you want a good visual go to www.woodht.homestead.com.
(My cieling is 7'5")
I agree with GHawk that a 15 foot seating distance works well. My second row of seats is an a 1 foot riser, 18 feet back. There's no problem seeing the screen from the back row.
My center channel is beneath the screen and projects well to both rows.
The relatively large screen adds to the immersive experience.