ideally, one would want a front wall over 18'. but many cannot afford that. so, you do what you can with what you have.
my front wall is 12', 8'8" screen width. while my speakers are not away from the side walls, they are not right up against them either. and toeing them in helps.
i don't claim to have the best setup around (i am sure Bryan could pick it apart). but it is a very nice setup for what it is.
what i recommend doing = shoot on the wall for a while. adjusting the image size. this will give you a much better idea of what you will like. or better yet, go to home depot/menards/whatever, and get a 4'x8' of white board. prop that up on the wall. it will HOT SPOT pretty badly, but it will give a good idea for sizing. then, after you decide, take the board back.