i would use:
<input type="hidden" name="on0_<?php echo $pkCart_Index+1; ?>" id="on0_<?php echo $pkCart_Index+1; ?>" value="<?php echo((isset($pkCart->size)):"Size":""); ?>"/>
<input type="hidden" name="os0_<?php echo $pkCart_Index+1; ?>" id="os0_<?php echo $pkCart_Index+1; ?>" value="<?php echo((isset($pkCart->size))?$pkCart->DisplayInfo("Size"):""); ?>"/>