<reference name="product_list_toolbar">
  <action method="addPagerLimit"><limit>20</limit></action>
  <!--action method="setDefaultGridPerPage"><limit>20</limit></action-->
  <!--action method="setDefaultListPerPage"><limit>20</limit></action-->
</reference>

<layout>
    <CATEGORY_13>
        <reference name="product_list_toolbar">
            <action method="addPagerLimit"><mode>grid</mode><limit>15</limit></action>
            <action method="addPagerLimit"><mode>list</mode><limit>15</limit></action>
        </reference>
    </CATEGORY_13>
    <CATEGORY_7>
        <reference name="product_list_toolbar">
            <action method="addPagerLimit"><mode>grid</mode><limit>30</limit></action>
            <action method="addPagerLimit"><mode>list</mode><limit>30</limit></action>
        </reference>
    </CATEGORY_7>
</layout>

<?php
class Namepace_Modulename_Model_Observer {

    /**
      *
      * Holds Categoy ids
      *
      * @var array
      *
    */
    protected $_categoryIds = array(13,7) ;

    /**
      *
      * Holds page Sizes
      *
      * @var array
      *
    */
    protected $_pageSize = array(
        13 => 15
        7  => 30
    ) ;

    /**
      *
      * Use to set Page Size
      *
      * @param Varint Object  | $observer
      *
      *
    */
    public function setPageSizeForCategory($observer)
    {
        $controller = $observer->getAction();
        $fullActionName = $controller->getFullActionName();
        $id = (int)$controller->getRequest()->getParam('id');

        //check whether current page is correspond to our special category. If not, returns
        if ($fullActionName == "catalog_category_view" && $id == in_array($this->_categoryIds)) {

            //check whether toolbar block exist or not
            $toolbar =  $controller->getLayout()->getBlock('product_list_toolbar');
            if($toolbar) {

                //sets page size to corresponding mode
                $listMode = $toolbar->getCurrentMode();
                $toolbar = $toolbar->addPagerLimit($listMode , $this->_pageSize[$id]);
            }

        }
        return;
    }
}