Manual:$wgUploadSizeWarning/en

Details
Warns the user if they attempt to upload a file larger than this value, specified in bytes. Setting  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 no need for this settings, as a red warning is shown prior and the upload will not be attempted.