The database will reserve the specified amount of memory for each record regardless of the values in that record. If you set up a state field with a length of 500 characters, and the user always puts in two letter abbreviations, the DBMS will 'pad' the field with 498 spaces. All of those spaces will be held in memory and on the disk each time the record is accessed, and waste a lot of valuable real estate on the disk and memory. Don't make fields any bigger than you have too, but don't make them too small.

The card catalog was actually 3 copies of the same database, sorted on different fields! The author catalog is sorted on the author field. (I don't have to tell you the others, do I?)

Because it makes an ideal key field. Every citizen in the United States has one, and they all are different. They also all have the same format. Social Security Numbers were never intended for this general use beyond government databases, but it has become very widespread.

The type of business, followed by the name of the business