<input type="file" id="myFile" />

//binds to onchange event of your input field
$('#myFile').bind('change', function() {

  //this.files[0].size gets the size of your file.
  alert(this.files[0].size);

});

$.validator.addMethod('filesize', function(value, element, param) {
    // param = size (en bytes)
    // element = element to validate (<input>)
    // value = value of the element (file name)
    return this.optional(element) || (element.files[0].size <= param)
});

$('#formid').validate({
    rules: { inputimage: { required: true, accept: "png|jpe?g|gif", filesize: 1048576  }},
    messages: { inputimage: "File must be JPG, GIF or PNG, less than 1MB" }
});