File Preview Solution for muse ( How do I add in width and height Restrictions?)

Right so i have this HTML code that I can embed into Muse which is -

<input type="file" accept="image/*" onchange="loadFile(event)">

<img id="output" />

<script>

    var loadFile = function(event) {

        var reader = new FileReader();

        reader.onload = function() {

            var output = document.getElementById('output');

            output.src = reader.result;

        };

        reader.readAsDataURL(event.target.files[0]);

    };

</script>

This code allows me to preview files but how do I add what width and height I want the file to be previewed at? Please help, many thanks!