Functions to add a parent count to WordPress nav menus. This is useful if you need to change nav element size based on the number of parent/top level elements.

add-nav-parent-count.php

<?php

/**
 * Gets the count of the parent items in a nav menu based upon the id specified.
 * 
 * @param 	string $nav_id The id of the nav item to get the parent count for.
 * @return 	bool|int False on fail, or the count of how many parent items there are.
 * @uses 	wp_get_nav_menu_items
 */
function count_nav_parent_items( $nav_id = null ) {

	// if we don't have a nav_location, exit
	if( $nav_id === null || $nav_id === '' )
		return false;

	// now get the nav items of the id of nav_location
	$items = wp_get_nav_menu_items( $nav_id );

	// if we don't have anything, exit
	if( count( $items ) <= 0 )
		return false;

	// go through each item
	$parents = array();
	foreach( $items as $item ) {

		// if the item's parent is 0, it's a top level, add it to an array
		if( $item->menu_item_parent == 0 )
			$parents[] = $item;
	}

	// return our count
	return count( $parents );
}

/**
 * Adds a class to a nav container based upon how many parent items it contains.
 * 
 * @param 	array $args The array of args supplied by the filter for the nav menu.
 * @return 	array Returns the same array passed in, just modified
 * @uses 	get_nav_menu_locations, count_nav_parent_items
 */
function add_nav_parent_count( $args = '' ) {
	
	// is the id in the args?
	if( isset( $args[ 'menu' ]->term_id ) && $args[ 'menu' ]->term_id !== 0 ) {

		// if so, set it
		$menu_id = $args[ 'menu' ]->term_id;

	// do we have a theme location?
	} elseif( isset( $args[ 'theme_location' ] ) && $args[ 'theme_location' ] !== '' ) {

		// get all the locations
		$theme_nav_locations = get_nav_menu_locations();

		// if we don't have any theme locations, exit
		if( count( $theme_nav_locations ) <= 0 ) {

			// add a theme locations not found class and exit
			$args[ 'menu_class' ] = $args[ 'menu_class' ] .' parent-items-tlnf';
			return $args;
		}

		// set the menu id
		$menu_id = $theme_nav_locations[ $args[ 'theme_location' ] ];

	// we have nothing
	} else {

		// add an id not found class and exit
		$args[ 'menu_class' ] = $args[ 'menu_class' ] .' parent-items-idnf';
		return $args;
	}

	// count the parents
	$parent_count = count_nav_parent_items( $menu_id );

	// if we got a count
	if( $parent_count ) {

		// add the class to the nav container
		$args[ 'menu_class' ] = $args[ 'menu_class' ] .' parent-items-'. $parent_count;

	// we didn't
	} else {

		// add a different class to the nav container
		$args[ 'menu_class' ] = $args[ 'menu_class' ] .' parent-items-pcnf';
	}

	// put the array back into play
	return $args;
}
add_filter( 'wp_nav_menu_args', 'add_nav_parent_count' );

Thanks for this. It wasn't exactly what I was looking for -- if menu item is a parent, assign a class to it -- but it put me in the right direction of creating an array of parents and adding items to it, then going through said array for each item. Thanks!