This script is for Illustrator CC. It resizes objects to have the same diagonal dimension.

BalanceDiagonal.jsx

// BalanceDiagonal.jsx
// 2017 Todd Linkner
// License: none (public domain)
// v1.0
//
// This script is for Illustrator CC. It resizes objects to have the same diagonal dimension.
//
// Install to /Applications/Adobe Illustrator CC/en_US/Scripts/Mobile

// bring Illustrator into focus

#target illustrator

var activeDoc = app.activeDocument,
    selection = activeDoc.selection;

if (activeDoc && selection.length == 1) {
  var w = selection[0].width,
      h = selection[0].height,
      hypotenuse = Math.sqrt(Math.pow(w, 2) + Math.pow(h, 2)),
      items = activeDoc.layers[0].pageItems
      ;
 for (i = items.length - 1; i >= 0; i--) {
      var w2 = items[i].width,
          h2 = items[i].height,
          // theta = atan(opposite/adjacent)
          angle = Math.atan(h2/w2)
          ;
      // adjacent = cos(angle) * hypotenuse
      items[i].width = Math.cos(angle) * hypotenuse;
      // opposite = sin(angle) * hypotenuse
      items[i].height = Math.sin(angle) * hypotenuse;
 }
 alert("Done.");
} else {
  alert("Error: Missing active document or selection or a single object. Try again.");
}