jQuery for handling toggle functionality with wp_localize_script ( small_menu_vars.size ) and WordPress.

sample-functions-child-theme.php

<?php
/**
 * krogs_child_theme_new_menu_size function.
 * 
 */
add_filter( 'krogs_theme_small_menu_size', 'krogs_child_theme_new_menu_size' );

function krogs_child_theme_new_menu_size( $size ) {

	$size = 450;

	return 450;

}

sample-functions-enqueue.php

<?php
/**
 * Registers scripts for the theme and enqueue those used sitewide.
 *
 * @since 0.1.0.
 */
add_action( 'wp_enqueue_scripts', 'krogs_theme_scripts' );

function krogs_theme_scripts() {

	wp_register_script( 'small-menu', get_template_directory_uri() . '/js/small-menu.js', array( 'jquery' ), '20120206', true );

	wp_localize_script('small-menu', 'small_menu_vars', array(
			'size' => apply_filters( 'krogs_theme_small_menu_size', 600 )
		)
	);

	wp_enqueue_script( 'small-menu' );

}

small-menu.js

/**
 * Handles toggling the main navigation menu for small screens.
 * small_menu_vars.size is used instead of "600" or other hard coded pixel value - it is defined by wp_localize_script in functions
 */
jQuery( document ).ready( function( $ ) {
	var $masthead = $( '#masthead' ),
	    timeout = false;

	$.fn.smallMenu = function() {
		$masthead.find( '.site-navigation' ).removeClass( 'main-navigation' ).addClass( 'main-small-navigation' );
		$masthead.find( '.site-navigation h1' ).removeClass( 'assistive-text' ).addClass( 'menu-toggle' );

		$( '.menu-toggle' ).click( function() {
			$masthead.find( '.menu' ).toggle();
			$( this ).toggleClass( 'toggled-on' );
		} );
	};

	// Check viewport width on first load.
	if ( $( window ).width() < small_menu_vars.size )
		$.fn.smallMenu();

	// Check viewport width when user resizes the browser window.
	$( window ).resize( function() {
		var browserWidth = $( window ).width();

		if ( false !== timeout )
			clearTimeout( timeout );

		timeout = setTimeout( function() {
			if ( browserWidth < small_menu_vars.size ) {
				$.fn.smallMenu();
			} else {
				$masthead.find( '.site-navigation' ).removeClass( 'main-small-navigation' ).addClass( 'main-navigation' );
				$masthead.find( '.site-navigation h1' ).removeClass( 'menu-toggle' ).addClass( 'assistive-text' );
				$masthead.find( '.menu' ).removeAttr( 'style' );
			}
		}, 200 );
	} );
} );