Can you give us more info to work with? The more you can tell us about the WHOLE setup, the better. Some variables will drive others, so it's good to keep things as flexible as possible upfront.
In my case, my last theater had similar room dimensions (24.25' long, 12.25' wide), two rows of reclinable seating (the rows were placed to optimize the 7.1 surround setup as much as possible, e.g., the rear row put eyes/ears about 4.1' from the rear wall, while the front row put eyes/ears about 12.1' from the screen, 12" riser for the rear row), with a 9" CRT front projector...and I ended up with a 56"x100" microperf 1.5 gain screen. In my case, regardless of room size, the projector served as the limiting factor for the size of the screen (and truth be told, I was pushing the limits there).
EDIT: I have attached a rough sample layout (showing screen, projector, seating, sight lines, etc.) of the above theater.
Take some tape and place it on the wall. Make an imaginary screen with the tape. You can look at different screen size and make a "screen" about that size with the tape. It will give you an idea of how big you want the screen to be. Decide how many rows of seats you are going to have. I have two in my room which is close to the size you have. If you are looking for a simple answer, I was told that for a room about the size of mine, a screen from 92 inch diagonal to about 110 inch would likely be the best choice. If you go with an acoustically transparent screen, check the Vutec Sound Screen Thread, or a perforated one, you can place the speakers behind the screen. The type of speakers that you have also play a role. I have large floor standing towers and did not want to place them all of the way in the corners. My screen size allows the speakers to be out from the corners some.
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