Area rugs are small, convenient rugs that are typically used to decorate a small space such as an entryway or hall. Standard sizes for these rugs range from two feet-by-three feet to four feet-by-six feet. The come in multiple shapes from round to square and rectangle.
Runners are long narrow carpets typically used along halls and stair cases. They can be as narrow as one foot wide, to four feet wide and be as long as you want.
Larger area rugs are typically used in rooms and come in a variety of standardized sizes. These sizes are 10 feet-by-10 feet and eight feet-by-eight feet. Less standard sizes are 10 feet-by-14 feet, nine feet-by-12 feet, eight feet-by-11 feet, seven feet-by-ten feet and six feet-by-nine feet.
Carpets also come in a variety of non-standard sizes. This is particularly common when the rug is hand-made or if it's a remnant. You can also have carpets made to a specific size that suits your space.