Given a 13'+ wide screen wall and a main viewing location of 18' I'd suggest you paint a screen of at least 122" diagonal (60" x 106") with the PJ placed at 13' from the screen. (...this allows for using Sintra to paint on. Drywall? OK...you can go for 130" diagonal @ 64" x 113" )
What with the end of the room being such a Boxed in affair, you can greatly...and I do mean greatly...improve the performance and presence of the image by painting out the side walls and ceiling to 5' in a much darker "Flat" Tan. This will be especially advantageous because if you plan on using the Screen as a "Sports-Bar" like affair with people up and moving about, playing Pool and such, because the Couch back's presence (...and a few heads...) will mean the Screen will have to be placed at least 36" off the floor. With a 64" tall screen and what appears to be 96" high wall, that will not really work...will it?
So...dropping down to 28" off the Floor...+ 64"= 88" leaving at minimum 8" between the Ceiling and the Screen.
That means the Ceiling needs to be darkened considerably at least 4-5' out from the screen wall.
RS-MaxxMudd v2 is a good choice...however if your saving the Mortgage Money by not opting for GOO, I'd suggest moving up a notch to Silver Fire v2.5 3.0
Spray with this Gun.http://www.gleempaint.com/noname.html
Let us know what you going to decide upon, size and paint wise and we'll go on from there.