If the end listener is going to be playing back in a small room, you should be mixing in a small room. If the end listener is going to be playing back in a large room, you should be mixing in a large room. The mixing room should emulate the playback environment.

This is why movies are mixed for theatrical presentation in a huge dubbing theatre that approximates the acoustics of a movie theatre, while TV shows intended to watch in your living room are mixed in a smaller control booth.

If you want to know what a proper monitoring situation should be, there is an ITU standard on the subject, ITU BS.1116-1 which talks about the basic requirements for a room for mixing material that will translate to typical living room playback. You should be able to find a copy of that standard with a google search.

Now, you SHOULD know that the way we think about big rooms and small rooms are different, because in a small room the reflections are perceived as frequency response changes, while in a larger room the time delay for those reflections is longer and so they are perceived as discrete echos. The F. Alton Everest book I cited earlier in this thread details the science here.
--scott