Manual:$wgUploadSizeWarning

Details
Warn the user if he attempts to upload a file larger than this value, specified in bytes. Setting $wgUploadSizeWarning to zero bytes will disable the warning. If triggered, the warning will inform the user that files are recommended to be smaller than this value. The user can choose to ignore the warning and upload the file.

Note: As of version 1.11.1, this warning is not given to the user before the file upload. If he attempts to upload a very large file, it will be a long time before he receives the warning. After the upload completes, if the user cancels, I assume MediaWiki will delete the file.