42" is nice for a bedroom, 55" is for a small room, a large room needs at least 80".
We have 46" touchscreens at work and that seems like a nice size for arm's length away. We had a 104" at our museum event this week. Ooh! Aah! I want one!
The big wide screens are too much for me. I have a 32", and I sit about 7 feet away -- perfect for me.
If I use the projector, I sit about 15-20 feet away -- I pretty much only use that for watching concert footage though.
The little tiny screens screw with my eyes, and the big ones screw with my eyes and brain. My eyes started going bonkers, when I was using dual 28" displays, two feet in front of me. After a couple years of that, I boxed them back up, because they were messing up my vision.
Be careful with your eyes...