An area rug should be big enough that it covers the majority of the floor you want it placed on. You should leave a 1-2 foot radius around the rug to show the floor that you currently have in place. You can use more than one area rug to cover more of the floor if necessary.
Some are just the size of the sitting area. Sofa coffee table and two chairs facing each other with a area rug under the coffee table. Others can take up 3/4 of the room leaving about a foot all around the room. This is always what you prefer and the size of the room and what it is used for.
An area rug size depends on your needs. Do you want to cover the entire room? Do you want to have a rug under just a table? Common sizes are 4x6, 5x8, 6x9, 9x12, and 12x15. Assess your needs to find the correct size.
In determining the size of an area rug, you must consider whether you want a room size or a small size. Allow 18"of bare floor space around the rug for a room size rug and have the furniture legs off
the rug for smaller size.