A rug can add so much to an interior, and is an extremely versatile furnishing item.
There are no hard or fast rules on how to position your rug. If you want to define/anchor an area in a large space, then a large rug with all the furniture on it is the best way to go. In a smaller room, the choice of pattern and size are really important to avoid a 'clumsy' look. Under a dining room table be sure not skimp on the size, allow a good margin around the table to allow chairs to be pulled back on - your rug needs to be approximately 1.2 - 1.5m wider and longer than your table (i.e 60-75cm around the perimeter of the table for the chairs to fit comfortably).
A rug can add so much to an interior, and is an extremely versatile furnishing item, as you can go either extremely bold or stay very subtle without doing too much else with other furniture/furnishings. Wool/natural fibre rugs will stay looking great for a long time if maintained well.