Edit in JSFiddle

$(function (){

    function format_float(num, pos)
    {
        var size = Math.pow(10, pos);
        return Math.round(num * size) / size;
    }

    function preview(input) {

        if (input.files && input.files[0]) {
            var reader = new FileReader();
            
            reader.onload = function (e) {
                $('.preview').attr('src', e.target.result);
                var KB = format_float(e.total / 1024, 2);
                $('.size').text("檔案大小：" + KB + " KB");
            }

            reader.readAsDataURL(input.files[0]);
        }
    }

    $("body").on("change", ".upl", function (){
        preview(this);
    })
    
})

<form>
    <input type='file' class="upl">
    <div>
        <img class="preview" style="max-width: 150px; max-height: 150px;">
        <div class="size"></div>
    </div>
</form>