You shouldn't need a $watch. Just bind to resize event on window:
'use strict';
var app = angular.module('plunker', []);
app.directive('myDirective', ['$window', function ($window) {
return {
link: link,
restrict: 'E',
template: '<div>window size: {{width}}px</div>'
};
function link(scope, element, attrs){
scope.width = $window.innerWidth;
angular.element($window).bind('resize', function(){
scope.width = $window.innerWidth;
// manuall $digest required as resize event
// is outside of angular
scope.$digest();
});
}
}]);