A king size mattress for a big bedroom is typically the largest that can be found in the common market.  It can be placed on top of a platform or straight on the floor.  There are four types of king size mattresses, king, California king, super king and grand king.  Only the king can be found outside Northern America and Europe.  All three except the super king can be found in any store.  A king size mattress is 76 inches wide and 80 inches long.  The California king is 72 inches wide and 84 inches long, limited to the United States and Canada and the grand king is 80 inches wide and 98 inches long, limited only to North America.  The super king is limited to Europe and is 72 inches wide and 78 inches long.  Tall people benefit from the extra room and the main benefit of the king size mattress is its size.