Configuring Log File Properties 
You can configure the maximum size of the log file, the maximum number of log files, and the name of the log file. By default, the maximum log file size is 10 MB and the maximum number of log files is 5. When a log file reaches its size limit, a new file is created, up to the maximum number of log files. After the maximum number of log files has been created, the first log file is deleted and replaced by a new one.
You can view log file properties in SAP NetWeaver Administrator, but to modify these properties you use the AS Java – Config Tool.
Open the AS Java – Config Tool (<INSTANCE>\j2ee\configtool\configtool.bat).
Under , you can modify the following:
Count: The maximum number of log files
Limit: The maximum size of each log file, in bytes
Pattern: The name of the log file
Save your changes and restart the portal.