Imsanity is a WordPress plugin by verysimple that automatically resizes insanely huge image uploads down to a more reasonable size. The plugin is configurable with a max width and height. Whenever a contributor uploads an image that is larger than the configured size, Imsanity will scale it down to the max size automatically and remove the original image.
This is the perfect plugin 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.
Automatically scales large image uploads to a more “sane” size
Allows configuration of max width/height
Once enabled, Imsanity requires no actions on the part of the user
Uses WordPress built-in image scaling functions
Download and info: http://wordpress.org/extend/plugins/imsanity/