Assess the size of your dining room: Your dining room size will dictate how large of an area rug you should use. Large dining rooms may need to utilize two or more area rugs based on multiple seating areas and tables. For smaller dining rooms consider using one that is larger than your dining room table, but smaller than the perimeter of the room. For those that have hardwood flooring, a border of the contrasting resilient flooring makes a nice contrast.
Defining a space: If you have an open area dining room that shares a space with a living room or kitchen area, an area rug will help you define the dining room without using walls. Choose a rug that uses colors from adjoining spaces but still defines the space. Therefore a mixture of neutrals from the surrounding rooms with a punch of an accent color to match the dining room chairs would be perfect.