The two things that matter most to keeping your eyes from over straining are keeping proper lighting in the TV-watching room and keeping a safe distance from the TV screen.
The Canadian Association of Optometrists did a study that proved it's equally as bad for your eye to watch TV in a bright room as it is in a dark one. Your everyday light is the best atmosphere for marathon movie sessions, and the CAO says that watching TV in optimal light can even have less strain on your eyes than reading a book.
The advice on a proper viewing distance for a certain size TV varies depending on where you look. But keep in mind that any figures you find on a manufacturer or retailer's Web site are probably skewed a bit to get you to pick up the biggest and most expensive TV they can.
The CAO recommends that a person watch TV from a distance of 5x the width of the picture (note that's the actual width of the screen—not the diagonal screen measure that's typically used to compare screen size), but anything more than 3x the width should be sufficient.