Resizes all images in a Google Docs document to a desired width, while limited to a max size (sizes are in cm).

google_docs_image_resizer.js

/**
 * @fileoverview Resizes all images in a document to a desired width
 * while limited to a max size. Sizes are in cm.
 *
 * This code is meant to be implemented as a script on a Google Docs
 * document: Tools > Script editor.
 *
 * Once open, paste the code, adjust your desired sizes and run:
 * `resizeToDesiredWidth`.
 *
 * Justification for the inconsistent and outdated syntax:
 * The current version of Apps Script does not fully support ES6.
 * As a result, this code is a mix of both. For example, `const`
 * are not allowed at global level, but they are within blocks
 * or functions. Similarly, functions declared as:
 * `function name() {}` are detected to be run by the script, but
 * `const name = function() {}` would not. Arrow functions or
 * nice loop logic are not implemented.
 */

/** Configure the desired width and max height of the images. */
var DESIRED_WIDTH_CM = 15;
var MAX_HEIGHT_CM = 8;

/** This PPI is based on my experience. It may differ for you. */
var PIXELS_PER_CM = 39;


/**
 * Resizes all images in the document based on desired and max size.
 */
function resizeToDesiredWidth() {
  const document = DocumentApp.getActiveDocument();
  const body = document.getBody();
  const images =  body.getImages();

  images.forEach(function(image) {
    const w = pxToCm(image.getWidth());
    const h = pxToCm(image.getHeight());
    
    const newDimensions = getDesiredImageSize(w, h);
    const newW = newDimensions[0];
    const newH = newDimensions[1];
    
    image.setWidth(cmToPx(newW));
    image.setHeight(cmToPx(newH));
  });
}

/**
 * Calculates new sizes of images adjusted to desired and max.
 * Image ratio is preserved.
 * Resize calculation is done to DESIRED_WIDTH_CM.
 * If image is over MAX_HEIGHT_CM, then image is resized to MAX_HEIGHT_CM.
 * @param {number} imageW Image width in cm.
 * @param {number} imageH Image height in cm.
 * @return {!Array<number>} New image dimensions: [newImageW, newImageH]
 */
var getDesiredImageSize = function(imageW, imageH) {
  const ratioW = imageW / DESIRED_WIDTH_CM;
  var newW = DESIRED_WIDTH_CM;
  var newH = imageH / ratioW;
  
  if (newH <= MAX_HEIGHT_CM) {
    return [newW, newH];
  }

  const ratioH = newH / MAX_HEIGHT_CM;
  newW = newW / ratioH;
  newH = MAX_HEIGHT_CM;
  return [newW, newH];
};

/**
 * Converts a measure in pixels to cm.
 * @param {number} measureInPx
 * @return {number} Equivalent measure in cm.
 */
var pxToCm = function(measureInPx) {
  return measureInPx / PIXELS_PER_CM;
};

/**
 * Converts a measure in cm to pixels.
 * @param {number} measureInCm
 * @return {number} Equivalent measure in pixels.
 */
var cmToPx = function(measureInCm) {
  return measureInCm * PIXELS_PER_CM;
};