My mistake

HTML

<div ng-controller="myCtrl">
    <div ng-repeat="usr in users | orderBy:'length'">
        {{usr.length}}: {{usr.0.foo}}
    </div>
</div>

JavaScript

var app = angular.module('myApp', []);

function myCtrl($scope) {
    $scope.users = {a: [
                {foo: 'bar'}, {foo: 'bar'}
            ], 
            b: [
                {foo: 'bay'}
            ], 
            c: [
                {foo: 'baz'}, {foo: 'baz'}, {foo: 'baz'}
            ]
      };
}