Its the viewing distance that matters not the size of the room, ok its 15' long but you sit like 5' from tv 46" would look wrong.
I sit 5' or so sometimes and TBH as long as I am using a good quality source then the experiance is even better. But then that is my preference.
As with all things related to TV's, I would 100% recommend trying it out for yourself if possible. Some people like LCD, others like plasma. Some people are OK with upscaled SD, others not so much. I got my screen set up in a certain way, other people come round and mention that theirs is set up differently. I know my dad has his setup differently.
Just varies from person to person.