Tapermonkey script that adds a text element to the top of a phabricator diff page with the total diff size. Doesn't have a +/- breakdown because that info isn't easy to access. This is to circumvent maintainers that refuse to add features, see https://secure.phabricator.com/T11768.

phabricator_lines_modified.js

// ==UserScript==
// @name         Show Total Number of Lines Modified in Diff
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Show total number of lines modified in diff at bottom of table of contents
// @author       lyahdav
// @match        https://phabricator.dropboxer.net/D*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    var files = document.getElementsByClassName('right aphront-table-view-nodevice');
    var totalLineCount = 0;
    for (var i=1;i<files.length; i++) {
        var file = files[i];
        var lineText = file.textContent;
        lineText = lineText.match(/(.+) lines/);
        if (lineText == null) { continue; }
        var lineCount = parseInt(lineText[1]);
        totalLineCount += lineCount;
    }

    var countElementContent = document.createTextNode('Total number of lines modified: ' + totalLineCount);
    var countElement = document.createElement('span');
    countElement.appendChild(countElementContent);
    countElement.style = 'font-size: 16px; color: rgb(151,234,151)';
    document.getElementsByClassName('phui-header-subheader')[0].appendChild(countElement);
    console.log(totalLineCount);

})();

Based off https://gist.github.com/lyahdav/30fabcfb2856a28d320114922c91a1e3, which only some small changes to work with the next element classnames as the phabricator UI has changed.