Automatically resize Wordpress images to a maximum width and height upon uploading?

I created a blog where some users can upload images through the Wordpress dashboard. The site gets bogged down quickly because the original images are so big. Some users don't have the knowledge to resize the images themselves before uploading them, and I don't want to have to resize them manually.

Is there any way I can set a maximum width and height for uploaded images? I don't even want the original to remain on the website. I want the largest version of the image on the website to match the width and height restrictions I set.

Add this code in your theme's functions.php it will replace the original image with the re-sized version.

function replace_uploaded_image($image_data) {
  // if there is no large image : return
  if (!isset($image_data['sizes']['large'])) return $image_data;

  // paths to the uploaded image and the large image
  $upload_dir = wp_upload_dir();
  $uploaded_image_location = $upload_dir['basedir'] . '/' .$image_data['file'];
  $large_image_location = $upload_dir['path'] . '/'.$image_data['sizes']['large']['file'];

  // delete the uploaded image
  unlink($uploaded_image_location);

  // rename the large image
  rename($large_image_location,$uploaded_image_location);

  // update image metadata and return them
  $image_data['width'] = $image_data['sizes']['large']['width'];
  $image_data['height'] = $image_data['sizes']['large']['height'];
  unset($image_data['sizes']['large']);

  return $image_data;
}

add_filter('wp_generate_attachment_metadata','replace_uploaded_image');

Source: Automatically replace original uploaded image with large image size