You can change the log file size from server.config.xml(located in <dynaTrace Folder>\server\conf )
By default, Correlation logs is 30MB. For server log and other logs this value is 10MB. And max log file number is 2 for Correlation logs and 5 for other logs.
Also, you can change the log level in debug mode (Press Ctrl+Shift+F9 in your AppMon client to activate debug mode) from Menu > Settings > dynaTrace Server > Settings > General > Debug Options.