I have a TV room 10 x 10 metres and it sounds great. With a sub, the effect is more room-filling. With more sensitive neighbours
the sub might be a little too loud. The volume can be adjusted. I wouldn't want to do without the sub. The film effects sound much better.

I can't really comment on the room size, I have 5x5x2.3 metres.
The sound: fantastic. Even without subwoofer
With a subwoofer it's just complete and gets the right atmosphere. The sub is very strong but also sounds very good at volume 4-6.