private Map<Integer, String> sizeIdNameMap = new HashMap<Integer, String>();

public class Product implements Comparable<Product>, Serializable {
       private long id;
       private long otherProductData;
       private Map<Integer, String> sizeIdNameMap = new HashMap<Integer, String>();
       //constructor;setters;getters;equals;hashCode
}
public class Size implements Comparable<Size>, Serializable {
       private long id;
       private long name;
       //constructor;setters;getters;equals;hashCode
}

<hibernate-mapping package="beans">
   <class name="Size" table="Size">
      <id name="id" column="id">
         <generator class="native" />
      </id>
      <property name="name" type="string" column="name" />
   </class>
   <class name="Product" table="Product">
      <id name="id" column="id">
         <generator class="native" />
      </id>
      <property name="otherProductData" type="string" not-null="true" length="200"/>
      <map name="sizeIdNameMap" table="ProductSizeJunction">
         <key column="personId"/>
         <map-key column="sizeId" type="integer" />
         <many-to-many ???????>
            ???????
         </many-to-many>
      </map>
   </class>
</hibernate-mapping>

public class SizeNamePropertyMap extends HashMap<Object, Object> {
   @Override
   public Object put(Object key, Object value) {
      String finalValue;
      if (value instanceof String){
         finalValue = (String) value;
      } else if (value instanceof Size){
         finalValue = ((Size) value).getName();
      } else {
         throw new RuntimeException("Class cast exception. The value is instance of:" +
               value.getClass().getName() + ". Should be either String or Size.");
      }
      return super.put(key, finalValue);
   }
}
public class Product implements Comparable<Product>, Serializable {
       private long id;
       private long otherProductData;
       private Map<Integer, Object> sizeIdNameMap = new SizeNamePropertyMap<Integer, String>();
       //constructor;setters;getters;equals;hashCode
}

hibernate mapping for the map element:
      <map name="sizeIdNameMap" table="ProductSizeJunction">
         <key column="personId"/>
         <map-key column="sizeId" type="integer" />
         <composite-element class="Size" >
            <property name="id" column="id" />
            <property name="name" column="name" />
         </composite-element>
      </map>