There is no one right size oriental rug for any given room. However, there are typical sizes which are used in certain areas of the home. Standard Oriental Rug Sizes here.
When choosing an Oriental Rug Size, keep in mind the following factors: Door clearances, floor vents, "centeredness" (is that a word?). A good way to get a feel for the space a rug may take up is to put down some newspaper in the size and shape of your soon to be rug.
Vestibules can typically handle 3x5 to 5x8 size rugs. Watch again for door clearances, as the height of a carpet may prohibit doors from being swung open!
living rooms are typically found to have 5x8, 6x9, 8x10, and in some cases (with some overlap of furniture) 9x12 and 10x14+. A great deal of the decision has to do with how much of the rug you wish to have underneath furniture. Some prefer to not kick up the edge of a rug when seated: get a large rug to go under the couches. Others prefer the rug to frame a coffee table and center the room: go with a smaller size such as 6x9.
Dining Rooms and bedrooms rugs most often are found to be at a minimum of 8x10. The more common sizes used seem to be 9x12, and 10x14. Remember, in the situation of a Dining Room, it's safer to go with a large rug. Distance created by the width and length of a dining room table can easily turn into the awkward "get the chair up onto the carpet."
The information provided above is for the typical home. If your house is large, go with bigger rugs!
If you have specific questions, feel free to contact us