Building a house to have a Home theatre. I like your style !!!
I guess the basics to consider are how many people would you like it to sit comfortably?
My converted cellar which i'm currently fitting out is a similar size, but its just myself and the misses mostly! There is squatting room on the floor though if it gets busy!
I've calculated a 70" image from just over 3m from the HS10 i have on order.
If you are going the pj route have alook at this site. It should give you rough idea of screen size from some of the best selling Pj's
http://www.projectorcentral.com/proj...=1861&metric=0
Best of luck with the build.
Mal