In the documentation I read that it is possible to constrain field content of single-line text fields, number fields and date fields. But how exactly do I do that?

My (simplified) Schema looks like this:

<!-- language: lang-xml -->

    <xsd:schema xmlns="uuid:2b" xmlns:tcmi="http://www.tridion.com/ContentManager/5.0/Instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" targetNamespace="uuid:2b">
      <xsd:import namespace="http://www.tridion.com/ContentManager/5.0/Instance"></xsd:import>
      <xsd:annotation>
        <xsd:appinfo>
          <tcm:Labels xmlns:tcm="http://www.tridion.com/ContentManager/5.0">
            <tcm:Label ElementName="tag" Metadata="false">Tag</tcm:Label>
          </tcm:Labels>
        </xsd:appinfo>
      </xsd:annotation>
      <xsd:element name="Content">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="tag" minOccurs="1" maxOccurs="unbounded" type="xsd:normalizedString">
              <xsd:annotation>
                <xsd:appinfo>
                  <tcm:ExtensionXml xmlns:tcm="http://www.tridion.com/ContentManager/5.0"></tcm:ExtensionXml>
                </xsd:appinfo>
              </xsd:annotation>
            </xsd:element>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:schema>

So how would I set a maximum length of 15 characters on the **tag** field?