based 64 image how to change height and width

Question

I have this image with a plugin to screen shot page using url and it creates based 64 image

<img src="data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs%3D" data-url="google.com" alt="google">

and this is my plug in

    $(window).load(function() {

        $('img[data-url]').each(function() {
            $.ajax({
                url: 'https://www.googleapis.com/pagespeedonline/v1/runPagespeed?url=' + $(this).data('url') + '&screenshot=true',
                context: this,
                type: 'GET',
                dataType: 'json',
                success: function(data) {
                    data = data.screenshot.data.replace(/_/g, '/').replace(/-/g, '+');
                    $(this).attr('src', 'data:image/jpeg;base64,' + data);
                }
            });
        });

});

when I view page source it shows that my image is 320 X 240 pixels, how to adjust it to make 500 x 500 pixels?

Appreciate if you can help me find out thanks, I search a lot but can't see solutions.