Here is my input fwiw. My room is a bit smaller at 11.5' x 14.75'. Originally I had a 120" screen which when almost from one wall to the other, and sat as far back in the room as possible. Although it was nice and big, I personally found it too big. A friend of mine actually got motion sickness playing some games on it... I am redoing the room and now I have a 92" screen. The nice thing is, now I can put my sofa a bit closer to the screen which makes placement of rear/side speakers possible. It also leaves room for my towers beside the screen (your room is wider so you wouldn't have this problem I don't think).
If I was only watching movies I would have done a 110" or so, but games I actually like the screen a tad smaller. Really you need to do what Brad suggested and wait on getting the screen until you have tried a few screen sizes in the room, otherwise you may regret your decision based on what other people like. I ordered a screen based on the max size that would fit on the wall and ended up regretting it, and getting a new one a year later.