|Default value||5 MB to 32 MB, depending on the size of the SGA, CPU count, and whether the operating system is 32-bit or 64-bit|
|Range of values||2 MB to operating system-dependent|
LOG_BUFFER specifies the amount of memory (in bytes) that Oracle uses when buffering redo entries to a redo log file. Redo log entries contain a record of the changes that have been made to the database block buffers. The LGWR process writes redo log entries from the log buffer to a redo log file.
The log buffer size depends on the number of redo strands in the system. One redo strand is allocated for every 16 CPUs and has a default size of 2 MB. Oracle allocates a minimum of 2 redo strands per instance. When the log buffer size is not specified, any remaining memory in the redo granules is given to the log buffer.
Oracle Database Performance Tuning Guide for more information on setting this parameter
Your operating system-specific Oracle documentation for the default value and range of values