Before you head out to go HDTV shopping, it's a good idea to take some measurements of the room in which you'll be viewing it.
First, measure the entire room size and write down the overall dimensions. This information will help you choose the size of your HDTV and any other components, such as speakers, that you may decide to purchase to go with the home theater system. Next, determine where you plan to place the television and measure that area. If you're going to place your new HDTV in a cabinet, take that into account, too. Finally, determine exactly where you plan to view the television from. Measure the area to the central seating position -- with the sofa or chairs perpendicular to the set. It's this measurement you will most likely use to choose your new HDTV.
Generally speaking, the closer you get to most television sets, the more the picture appears to deteriorate. The human eye picks up imperfections when you're up close. With an HDTV, however, you can sit closer and not lose the picture quality that you would watching a standard-definition set.
Why is this important? When you get into the expansive world of a big-box retail store, the HDTVs appear much smaller. If you pick one on the fly, you may end up with a gigantic set which won't look right in your home watching environment.
You can find a cheat sheet at CNET that will help you when you go shopping.