I suspect you may need to do this outside of FileMaker unless you can find a way of capturing that metadata of the image before you pass it to SuperContainer. eg with AppleScript but you seem to be on the Windows platform.
I then wondered about loading the image into a temporary container field where that data could be extracted. But the GetLayoutObjectAttribute function does not seen to have an attribute for document size.
SuperContainer does, however, "know" the file size as that data can be displayed with the info command. Thus you may be able to extract that data from the WebViewer and subsequently use it to delete that offending file, and its parent folder, from the storage location.
That's all I could think of.
You can also modify the web server to limit the upload directly. If you view our documentation at:
You'll find instructions on how to accomplish this for Apache and IIS. Please let me know if you have any further questions or need clarification on anything!