You can handle Session.FileTransferProgress event and set FileTransferProgressEventArgs.Cancel to abort the transfer once you download sufficient data.
https://winscp.net/eng/docs/library_session_filetransferprogress
https://winscp.net/eng/docs/library_filetransferprogresseventargs

A request for streaming interface is tracked here:
https://winscp.net/tracker/1738

Is it possible to read only one line per ftp without first downloading the whole file with c#?
With fluentFtp I did that by opening a stream and reading a line and then sending an ABOR command.
But WinScp does not support streams. Does anyone have another idea?