static Map<Class, Int> classSizeInBytes_cache = weakHashMap();

static int classSizeInBytes(Class c) {
  Int size = classSizeInBytes_cache.get(c);
  if (size == null)
    classSizeInBytes_cache.put(c, size = classSizeInBytes_uncached(c));
  ret size;
}

static int classSizeInBytes_uncached(Class c) ctex {
  L<File> files = cast get(getClassLoader(c), 'files);
  S name = c.getName().replace('.', '/') + ".class";
  for (File location : files) 
    if (location.isDirectory()) {
      File f = new File(location, name);
      if (f.exists()) ret toInt_checked(fileSize(f));
    } else if (location.isFile()) pcall {
      temp ZipFile zip = new ZipFile(location);
      ZipEntry entry = zip.getEntry(name);
      if (entry != null) ret toInt_checked(entry.getSize());
    }
  print(name + " not found in: " + files);
  ret -1;
}

Began life as a copy of #1012231