Screen size help for a noob

Hi fellas, this is gonna be my first home theater system and Im lost :P, Im planning on getting the panasonic AE7000U as a projector ( if there is anything better for that price range please do recommend ) and Im not sure what screen size to get, the demensions of the room is as follows:

length:26.25 feet
width:23.3 feet
hight :9.8 feet
wall where screen is gonna be:13 feet ( of usable space its actually 16.4 feet)

seating distance from screen: 15-16 feet depending on ur recommendation

and I have alot of light since this is my living room, and I have huge windows for the balcony, I put dark shutters but I still have some light shining through soo I cant control that part of my setup.

any help would be much apreciated for screen size and the type of screen I should get for lets call it ambient light situations :P

cheers,
manaf

forgot to mention that it has to be motorized as it will drop down and cover the tv for when we want to use it

thinking 16:9 and I will be using it for mostly movies and 3D, maybe watch UFC on it or soccer every once in a while. I really cant move the furniture or else the wife would kill me :P, so Im trying to make due with what I can, what do u sugest other than the panasonic, here a few pics of the room Im gonna be working with.

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ps I can go with a bigger screen if need be and the wall in the first pic is now flat

do they make motorized black diamond ??