The size of mattress will greatly depend on the person(s) who is using it.  If you are single, you might like a twin or full size mattress.  For a couple you’ll want at least a queen size mattress, but more likely a king size mattress set.
Keep in mind, the size of your room and the amount of space allocated for the bed. A king size is 76” x 80” and can be too large for some rooms. If you are unsure, it’s wise to take a measurement beforehand.
A twin accommodates one person and is most commonly used for daybeds, trundles, and children’s beds.  A full grown adult may find that a twin mattress is a tight fit.
Most commonly seen in college dorm rooms, twin extra long mattresses provide excellent length while at the same time saving space in cramped quarters.
A full (aka double, regular, standard) is the recommended size to comfortably accommodate one adult.  Two individuals might find a full size mattress a bit snug, as each person will have much less than a twin size mattress each.  This size is perfect for an older child.
A queen is America’s most commonly used mattress size.  Equal in length but not nearly as wide as the King, a queen is a great fit for most master bedrooms.  Crammed for space? Have narrow doorways and staircases?  Queen box springs are available in two piesces (split box springs) for easy moving..
A king (aka Eastern King) is quite a bit wider than a queen, offering each sleeper the equivalent to a twin size mattress.  This bed is well-suited for a large master bedroom.  This mattress comes standard with two twinxl size box springs.

The California King (aka Western King) is the standard size king on the West Coast. The longest mattress by four inches, the California King is comfortable and spacious for all adults. Comes standard with two box springs.