The use these elements:

• length - the exact length of the lexical value.

• minLength - the minimum length of the lexical value.

• maxLength - the maximum length of the lexical value.

The maximum must always be bigger than the minimum.

A string with two characters

<xs:simpleType name="code">
   <xs:restriction base="xs:string">
      <xs:length value="2"/>
   </xs:restriction>
</xs:simpleType>