Toao.net

Sometimes, it’s not necessary to keep copies of full resolution images on your web server. But, when you solicit images from many sources, you may not be able to rely on users to size images to your specifications. Here’s a very simple plugin that removes the full resolution image and replaces it with an image resized to your preference.
The plugin makes no changes to your database and is written with just 17 lines of code.

Maximum Image Size

When extremely large images are uploaded to the Media Library, replace it with the Large size image, configured in Settings >> Media.

Sean Butze

Many users do not compress or resize their images before uploading them into a post, and oftentimes WordPress’s maximum upload limit of 2MB-10MB is still too large to prevent the insertion of photos that can signficantly slow down a website.
WP Image Size Limit allows an administrator to set a custom file size limit that is specific to image files and smaller than WordPress’s general file size limit.
This is especially useful when you need to put tighter restriction on image uploads but want to preserve the ability to upload larger files of other formats (audio, video, etc.).

WP Image Size Limit

Just what I needed to tamp down those designers who think it's okay to use 20MB images!

Brilliant

Adds a new setting under Settings -> Media where an admin can set a maximum upload file size for image files.

Maximum Image Size vs WP Image Size Limit