ImageMinSize module allows you to define a minimum width and height an image needs to have when uploading. This is same as the already existing maximum image size setting, just on the other end. You can set the max and min width and height setting to the same to restrict uploaded images to a specific size. If the requirements aren't met it will delete the image and show an error.
How to install:
Download the contents of this repository and put the folder renamed as "ImageMinSize" into your site/modules/ folder
Login to processwire and got to Modules page and click "Check for new modules". You should see a note that a new modules was found. Install the ImageMinSize module under "Image" section.
You now have additional settings for minimum size on image fields.