Esrender uses Apache's log4php library. It's configuration-file lives under
conf/de.metaventis.esrender.log4php.properties
Its most important lines are
log4php.rootLogger
to set the log-verbosity, and
log4php.appender.default.file
to setup the path of the resulting log-file.
Esrender does NOT provide log-rotation ATM, so please install this using your favorite log-rotation-manager.