Oversized king bedspreads are 114 inches by 120 inches for a California king bed and 120 inches by 120 inches for an Eastern king bed. The standard king size mattress is 76 inches by 80 inches, so any bed size over this is an oversized king.
The proper size of a bedspread also depends on how high the bed is. Even a standard king size mattress may need an oversized bedspread if it is on risers. The suggested comforter dimensions for an oversized king bed are 102 inches by 86 inches or 102 inches by 94 inches. A blanket for these beds is usually 108 inches by 90 inches.