Manual:$wgUploadSizeWarning

Details
Warns 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.

As of version 1.21+/- there is not need for this settings, as a red warning is shown prior and the selected file is not even attempting to upload.