JavaScript file size validation for HTML 5, using jQuery Tools validation and Parsley validation. (Additionally, extended modernizer for HTML5 File API.). The 'data-filemaxsize' attribute should be set in megabytes.

file_size_validation.jqtools.js

/**
 * File size validation using jQuery Tools Validation (http://jquerytools.org/documentation/validator/index.html)
 */
if (Modernizr.fileapi) {
  $.tools.validator.fn('input[type="file"][data-filemaxsize]', function($file_input, value) {
    var files = $file_input.get(0).files, 
        max_megabytes = parseInt($file_input.data('filemaxsize')),
        max_bytes = max_megabytes * BYTES_PER_MEGABYTE;

    if (files.length > 0 && files[0].size > max_bytes) {
      return "The selected file can not be larger than " + max_megabytes + " megabytes"
    }

    return true;
  });
}


$('form').validator({ 
  // Need to listen for 'change' event so error will disappear when user selects a file with a valid size.
  errorInputEvent: 'keyup change'
});

file_size_validator.parsley.js

/**
 * File size validation using Parsley JS Validation (http://parsleyjs.org)
 */
$('form').parsley({
  validators: {
    filemaxsize: function (val, max_megabytes, parsleyField) {
      if (!Modernizr.fileapi) { return true; }

      var $file_input = $(parsleyField.element);
      if ($file_input.is(':not(input[type="file"])')) {
        console.log("Validation on max file size only works on file input types");
        return true;
      }

      var max_bytes = max_megabytes * BYTES_PER_MEGABYTE, files = $file_input.get(0).files;

      if (files.length == 0) {
        // No file, so valid. (Required check should ensure file is selected)
        return true; 
      }

      return files.length == 1  && files[0].size <= max_bytes;
    }
  },
  messages: {
    filemaxsize: "The file cannot be more than %s megabytes."
  }
});

modernizr.fileapi.js

/**
 * Extension to Modernizer for File API support
 */
window.Modernizr.addTest('fileapi', function() { return window.File && window.FileReader; });

window.BYTES_PER_MEGABYTE = 1048576;