How to FTP 'get' a partial file only

Use curl. From the man page:

   -r/--range <range>
          (HTTP/FTP/FILE)  Retrieve  a byte range (i.e a partial document)
          from a HTTP/1.1, FTP server or a local FILE. Ranges can be spec-
          ified in a number of ways.

          0-499     specifies the first 500 bytes

          500-999   specifies the second 500 bytes

          -500      specifies the last 500 bytes

          9500-     specifies the bytes from offset 9500 and forward

          0-0,-1    specifies the first and last byte only(*)(H)

          500-700,600-799
                    specifies 300 bytes from offset 500(H)

However, note that the SIZE extension must be supported by the server for this to work.

I think you want to use curl for this

From the man page:

   -r/--range <range>
          (HTTP/FTP/FILE)  Retrieve  a byte range (i.e a partial document)
          from a HTTP/1.1, FTP server or a local FILE. Ranges can be spec‐
          ified in a number of ways.

          0-499     specifies the first 500 bytes

          500-999   specifies the second 500 bytes

          -500      specifies the last 500 bytes

          9500-     specifies the bytes from offset 9500 and forward