The logs can be rotated based on the size and it can be achieved through a very slight change in portal-log4j.xml. This file can be found in \webapps\ROOT\WEB-INF\lib\portal-impl.jar\META-INF\. Also, to make changes in the default file, it is suggested to create a portal-log4j-ext.xml and insert the desired changes there, to avoid making a permanent change to the original file and place it in this location ${TOMCAT_HOME}/webapps/ROOT/WEB-INF/classes/META-INF.

log4j.appender.DAILY=org.apache.log4j.DailyRollingFileAppender
log4j.appender.DAILY.File=/logs/liferay.log
log4j.appender.DAILY.DatePattern='.'yyyy-MM-dd
log4j.appender.DAILY.layout=org.apache.log4j.PatternLayout
log4j.appender.DAILY.layout.conversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS} [%p] %c:%L - %m%n
log4j.appender.DAILY.MaxFileSize=100KB
log4j.appender.DAILY.MaxBackupIndex=5

Note: The file is rolled over after a specified size has been reached. i.e. This log4j.properties file will create a log file named liferay.log in the folder /logs. If this file exceeds a size of 100KB, it will be renamed to liferay.log.1 and replaced by a new liferay.log file. A maximum of 5 such backup files will be created. To configure max file size in megabytes, use MB as in 20MB. Please reach out to the Liferay Global Services team for further help.

https://stackoverflow.com/questions/7994015/how-to-control-log-file-with-daily-rolling-and-max-file-size-by-log4j 

https://logging.apache.org/log4php/docs/appenders/rolling-file.html