I don't think it would be to big in that size room by anyones standards ( I would look into the 92").
I would start by getting the screen size and making a carboard pattern. If your setting long-ways in the room, then consider how far the tv and seating will be from the walls. Then see how small it looks at that distance (Even at 18', face to tv, it's going to seem far away).That will get you started. Then consider when you go to the theater, do like to sit in the front, middle, or rear. For 3D, you are going to need to be a lot closer, so consider if you can move the furnature for those times. Hope this helps.
remember that the minimum seating distance for watching HD program material is 1.5 times the screen width away from the screen. This is the minimum distance at which individual pixels will not be visible. Sitting at the minimum distance is especially important for 3D viewing as it brings the screen edges closer to the periphery of your vision, greatly enhancing the ability of 3d images to come toward you. Sitting any closer than the minimum distance will begin to reveal individual pixels and motion artifacts. In a perfect world, where all program material was in HD (we're getting there...) you would place your seating approximately ten and a half feet away from the 82" screen and no closer. There really is no limit as to how far away can you sit. For standard definition programming (as is still found abundantly on cable and satellite providers) the appropriate distance would be nearly double the amount required for viewing HD material. This is because SD programming will show pixels and motion artifacting (combing, judder, etc.) at closer distances. I placed my 50" Samsung DLP (which I love) at a distance of ten feet from the couch. I've recently gotten rid of my cable boxes altogether and either watch Blu-rays(including 3D) or Netflix streaming, which is mostly in HD now. I would like to sit closer, but I can't really have the couch six feet from the screen in our living room. The alternative is to get a bigger screen and keep the couch where it is, or move the recliner real close to the TV when I want to watch Tron in 3D by myself (when the wife isn't looking...) Small indulgence
Your primary consideration needs to be where you are intending to place your seating and the type of programming you mainly intend to watch. If you still watch alot of SD programming I recommend sitting at 2 1/2 - 3X the screen width away, otherwise you will not be happy with the picture as it will look grainy. So just pick the screen size that matches your needs.
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