Dedicated home theatre suggestions

[lokipei]

We are looking to build our first home in about 6-12 months time and want to include a home theatre as part of the package. We are looking at the back wall being about 4m (about 13 feet) or may be as close as 3.8m(just over 12 feet) depending on the design. Some go back about 4.5 metres but would mean sacrifiing other areas of the house. The front wall will probably be similar dimensions. Anything bigger is possibly out of our budget.
I currently have a 65inch LG 2013 4K tv which we will probably keep in the lounger room for tv, but looking at getting a projector from the roof for the theatre, with a 120 inch 2:35 screen.
Would this be too big for this room?
Also would like 7.1 surround, however I'm not sure if we would have room for the speakers at the back. Is it possible to install the back speakers from the roof, or would this not be effective?