https://wordpress.org/plugins/imsanity/

Imsanity automatically resizes huge image uploads down to a size that is more reasonable for display in browser, yet still more than large enough for typical website use. The plugin is configurable with a max width, height and quality. When a contributor uploads an image that is larger than the configured size, Imsanity will automatically scale it down to the configured size and replace the original image.

Imsanity also provides a bulk-resize feature to selectively resize previously uploaded images to free up disk space.

This plugin is ideal for blogs that do not require hi-resolution original images to be stored and/or the contributors don’t want (or understand how) to scale images before uploading.

Features

• Automatically scales large image uploads to a more “sane” size
• Bulk-resize feature to selectively resize existing images
• Allows configuration of max width/height and jpg quality
• Optionally converts BMP files to JPG so image can be scaled
• Once enabled, Imsanity requires no actions on the part of the user
• Uses WordPress built-in image scaling functions