1, Changing the default size of the description field is not easy - you will need a template override to do this. Though most editors remember the size change if you re-size the edit area when you edit an event
2. Assuming you are using Joomla 3.3 - create a file templates/YOURTEMPLATE/html/com_jevents/icalevent/edit_datetime.php with this content
<?php
defined('JPATH_BASE') or die();
// is it a new event
if($this->row->ev_id()==0){
// make it an all day event
$this->row->_alldayevent = 1;
// to set no end time use this
//$this->row->_noendtime = 1;
$document = JFactory::getDocument();
$document->addScriptDeclaration("
window.addEvent('load', function(){
$('allDayEvent').checked=true;
toggleAllDayEvent();
});
");
}
include(JEV_ADMINPATH."/views/icalevent/tmpl/".basename(__FILE__));