There is a formula for sizing the flat panel, keyed to the size of the room and your normal viewing distance. Big screens don't look so great if you're too close, but you should still go as big as the formula recommends.

The store will probably only show HD content. Try to watch a standard broadcast program, since most TV is still 480i (well at least Comcast is), and see how well the set handles it. My Sony Bravia shows great HD, but some standard broadcasts look like old video tape.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma