We have a room that is about 13 x 10'. We project on the 10' axis, and the screen is 90". I could get a few more inches out of the screen, but I am projecting from a weird place (under the sofa from the side and have to keep the screen slightly smaller than all out).
We project directly on the wall and the picture is great!
I think you may find when you use the screen/distance calculator that if you are sitting around the same distance (or less) from the screen than the projector you will have a large viewing area. 90" at 9 feet is huge even if the screen isn't as big as a 120" screen. The first couple of weeks my eyes were sore because they are not used to such a large screen.
Also a 90" screen is the same area as 4 x 45" screens...and is much brighter than the same projector at 120".
It is definitely worth it to have a projector even in a small room. The only problem I've found is that I can't have more than 3 people watching a movie or else we run out of room. If that is not a problem, go for it!