Heres a little calculator you can use...http://www.projectorcentral.com/InFocus_Ho...culator-pro.htm
Im familiar with projector setups as Ive had a few.
The first thing you need to do is to figure out screen size. My room isnt very wide so I got 77" diagonal screen instead of the norm which is 92"(the picture looks far better on the 77" anyways).
The common thing to do is to double the diagonal measurement of your screen, and thats what determines where the projector goes(distance).
My projector is 13ft from the screen, and was recommended that it goes between 10.5 - 13.5 away, you have a few ft to work with. 77" screen size x 2 is 154". Mines 156 inches away from screen.
If you have a decent wall, dark white, grey(not paneling), youd be surprised at how good it looks. You can make the image small if you move the projector closer.
A friend brings his DLP projector over for Lan parties, props it up on a coffee table, puts it about 6 ft from my back wall and he gets a 40" to 50" screen