From the 8.1.7 Utilities Manual, the chapter on Export
Default: operating system-dependent
Specifies the length, in bytes, of the file record. The RECORDLENGTH parameter is necessary when you must transfer the export file to another operating system that uses a different default value.
If you do not define this parameter, it defaults to your platform-dependent value for BUFSIZ. For more information about the BUFSIZ default value, see your operating system-specific documentation.
You can set RECORDLENGTH to any value equal to or greater than your system’s BUFSIZ. (The highest value is 64KB.) Changing the RECORDLENGTH parameter affects only the size of data that accumulates before writing to the disk. It does not affect the operating system file block size.
Note: You can use this parameter to specify the size of the Export I/O buffer.
Additional Information: See your Oracle operating system-specific documentation to determine the proper value or to create a file with a different record size.