functions.php

<?php

/* 
 * Equal Sized Menu Items
 * @author Bill Erickson
 * @link http://www.billerickson.net/menu-item-equal-widths/
 *
 * @param array $classes, current menu classes
 * @param object $item, current menu item
 * @param object $args, menu arguments
 * @return array $classes
 */
function be_equal_sized_menu_items( $classes, $item, $args ) {
	
	// Only run on the header menu
	if( 'header' !== $args->theme_location )
		return $classes;
	
	// Only run on top level menu items
	if( $item->menu_item_parent )
		return $classes;
	
	// Grab all menu items	
	$menu_items = wp_get_nav_menu_items( $args->menu->term_id );
	
	// Remove submenu items
	foreach( $menu_items as $i => $menu_item )
		if( $menu_item->menu_item_parent )
			unset( $menu_items[$i] );
	
	// Count all top level items, and add as a class
	$classes[] = 'count-' . count( $menu_items );
	return $classes;

}
add_filter( 'nav_menu_css_class', 'be_equal_sized_menu_items', 10, 3 );

style.css

.genesis-nav-menu .menu-item.count-2 {
	width: 50%;
}

.genesis-nav-menu .menu-item.count-3 {
	width: 33.3333%;
}

.genesis-nav-menu .menu-item.count-4 {
	width: 25%;
}

.genesis-nav-menu .menu-item.count-5 {
	width: 20%;
}

.genesis-nav-menu .menu-item.count-6 {
	width: 16.666666667%;
}

.genesis-nav-menu .menu-item.count-7 {
	width: 14.285714286%;
}

.genesis-nav-menu .menu-item.count-8 {
	width: 12.5%;
}

.genesis-nav-menu .menu-item.count-9 {
	width: 11.111111111%;
}

.genesis-nav-menu .menu-item.count-10 {
	width: 10%;
}