This worked for me on the Classic Designer on Forms version 11.0.2212.30907.

This works by adding a new parsley validation to the upload field that totals the size of all attached files, and if it exceeds the size, it'll give the error.  To set it up, give your upload field(s) the CSS Class Name of maxUploadSize, and then set the max size in the Javascript on line 4.  The only thing I could never get working was to have it validate as soon as the upload happens instead of at form submission.

$(document).ready(function() {
  
  //Set the max file size for the upload validation (in MB).
  var maxTotalFileSizeAllowed = 5;
  
  //Add parsley validator for max total file size of uploaded files.
  window.Parsley.addValidator('maxtotalfilesize', {
    validateString: function(value, requirement, parsleyField) {
      var totalSize = 0;
      parsleyField.$element.parent().find('.files .imgSize').each(function() {
        var length = $(this).attr('title').toString();
        var size = parseFloat(length.substring(0, length.length - 2));
        var scale = length.substring(length.length - 2);
        if(scale == 'KB') { size = size * 1024; }
        else if(scale == 'MB') { size = size * 1024 * 1024; }
        else if(scale == 'GB') { size = size * 1024 * 1024 * 1024; }
        else if(scale == 'TB') { size = size * 1024 * 1024 * 1024 * 1024; }
        totalSize = totalSize + size;
      });
      return totalSize <= parseFloat(requirement * 1024 * 1024);
    },
    messages: {
      en: 'The total size of all files uploaded should not exceed ' + maxTotalFileSizeAllowed + ' MB.'
    }
  });
  
  //Add the custom parsley validation to the file upload that has the maxUploadSize class.
  $('.maxUploadSize input').attr('data-parsley-maxtotalfilesize', maxTotalFileSizeAllowed);
  
});