How does the StringLengthAttribute work?

Keyword as follows:

 class PartMetadata
 {
     // required keyword forces user to enter input
     [Required] 
     [DisplayName("Part number")]
     // or [Required(ErrorMessage="* Required")]

     // ErrorMessage in string only enforces when data is entered
     [StringLength(50, MinimumLength = 3, ErrorMessage = "* Part numbers must be between 3 and 50 character in length.")]
     public string Number { get; set; }

Yes, that is the correct behavior. StringLength verifies that a string is a certain length, but does not REQUIRE that the string be entered. Decorate Description with [Required], so that you have both a requirement for the string, and StringLength will provide the constraints on the string length.