Choosing the right size rug...In most cases people generally feel they can "get away with" a smaller size, and often because it costs a little less. In most cases however, that's not the best idea.
If you choose a rug that is too small under a dining table, the chair legs will catch the rug. Too small in a lounge or family area and the room will feel smaller and more cluttered. Too small under the bed and your feet will be on the cold tiles or timber in the morning. There are many impacts if you buy a rug that is too small.
If you follow the direction and inspiration of most well-known decorators and designers, they will tell you that putting a larger size rug in a room really makes a space feel bigger and less cluttered. It can also help define an area such as the separation between a lounge and family or dining area. When you do decide a size for your space, usually the rug you need will actually be one size larger than you planned.
Remember when you're shopping in a store and a rug is hanging, it looks a LOT smaller when it's on the ground.
Feel free to use this chart below that will give you some idea of what's ideal, as well as photos of rug sizes in spaces to help you see these points.