In types.xsd:

<xs:simpleType name="Sizes">
   <xs:restriction base="xs:postiveIntegers"/>
</xs:simpleType>


<xs:element name="size" type="my:Sizes"/>

The redefine:

<xs:redefine schemaLocation="types.xsd">
  <xs:simpleType name="Sizes">
  <xs:restriction base="my:Sizes">
    <xs:enumeration value="2"/>
    <xs:enumeration value="4"/>
    <xs:enumeration value="6"/>
    <xs:enumeration value="8"/>
    <xs:enumeration value="10"/>
    <xs:enumeration value="12"/>
  </xs:restriction>
  </xs:simpleType>
</xs:redefine>

The effect is the element 'size' can only have content of '2', '4',...,'12'.