Rugs come in all sorts of sizes but I include some of the most popular ones in this post. A very common mistake is selecting a rug that is too small for the space, for example, I have never seen an 5' x 8' rug work in a living room layout. The easiest rule to keep in mind is that the front legs of the main furniture (sofa, chairs) must touch the rug and there should be a minimum of six inches of rug on either side of each furniture piece. Side tables and lamps do not have to sit on the rug but it's not a bad thing if they do.
Side tables do not need to sit on the rug but should at least be within six inches of the rug. A good rule of thumb is that the amount of rug at the end of the bed should be (at least) the same amount shown on both sides.