Client side Image Resizing

To do client-side image resizing you need to include the following scripts:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.1/jquery.min.js"></script>
<!-- The jQuery UI widget factory, can be omitted if jQuery UI is already included -->
<script src="js/vendor/jquery.ui.widget.js"></script>
<!-- The Load Image plugin is included for the preview images and image resizing functionality -->
<script src="http://blueimp.github.io/JavaScript-Load-Image/js/load-image.min.js"></script>
<!-- The Canvas to Blob plugin is included for image resizing functionality -->
<script src="http://blueimp.github.io/JavaScript-Canvas-to-Blob/js/canvas-to-blob.min.js"></script>
<!-- The Iframe Transport is required for browsers without support for XHR file uploads -->
<script src="js/jquery.iframe-transport.js"></script>
<!-- The basic File Upload plugin -->
<script src="js/jquery.fileupload.js"></script>
<!-- The File Upload processing plugin -->
<script src="js/jquery.fileupload-process.js"></script>
<!-- The File Upload image preview & resize plugin -->
<script src="js/jquery.fileupload-image.js"></script>

Please note that you should download and upload all resources to your own server and not reference source files from the blueimp.github.io domain.

Then, all you need to do is to set the option disableImageResize to false.
By default, images are resized to FullHD (1920x1080), but you can define your own Options:

$('#fileupload').fileupload({
    url: '//jquery-file-upload.appspot.com/',
    dataType: 'json',
    // Enable image resizing, except for Android and Opera,
    // which actually support image resizing, but fail to
    // send Blob objects via XHR requests:
    disableImageResize: /Android(?!.*Chrome)|Opera/
        .test(window.navigator && navigator.userAgent),
    imageMaxWidth: 800,
    imageMaxHeight: 800,
    imageCrop: true // Force cropped images
})