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  Function 23H (decimal 35) reports the size of a file in terms of the
  number of records in the file. DS:DX points to the FCB of the file you
  want to know about. Before calling the function, the FCB should be left
  unopened and the record-size field in the FCB filled in. If you set the
  record size to 1, the file size is reported in bytes, which is most likely
  what you want.

  If the operation is successful, AL = 00H and the file size is inserted
  into the FCB. If the file is not found, AL = FFH.