Living Area
In a living space, a rug should be centered under your coffee table and chosen with the furniture arrangement in mind. Living room seating can be positioned in a number of ways: with four legs on the rug, with four legs off of the rug, or with only the front two legs on the rug. Good design dictates a uniform choice, so if only the front legs of your sofa rest on the area rug, the same should go for the armchair, too. Typically, living area rugs are 5x8, 8x10, or 9x12 feet.
Dining Room
No matter what, choose a rug that’s larger than the size of your table. To prevent chairs from rocking on an uneven surface, the area rug under your dining table should be big enough to account for the distance that dining chairs get pushed back from the table when people sit or stand. To find the ideal size, add 36 inches to both the length and width of your table (or 36 inches to the diameter if it’s round).
Bedroom
Give your feet a soft surface to land on every morning by choosing an area rug that extends 18 to 24 inches beyond each side of the bed. If your bed is flanked by nightstands, measure the distance from outer edge to edge of both nightstands, and use this width as your guide. When determining the length of the rug, account for any furniture (like a bench or pair of ottomans) at the foot of the bed: All four legs of the piece should either fit on or off the rug.
Credit: https://www.jossandmain.com/inspiration/how-to-buy-an-area-rug-S12495.html
