You can define the type in the list.

Example:

<xs:simpleType name="MenShoeSizesUS">
   <xs:list>
      <xs:simpleType>
         <xs:restriction base="xs:integer">
            <xs:minInclusive value="7"/>
            <xs:maxInclusive value="14"/>
         </xs:restriction>
      </xs:simpleType>
   </xs:list>
</xs:simpleType>

An instance:

9 10 14