I'm going to make a guess that you are running Linux, that you run JBoss AS as a service (which version are you running) and that the stderrwfc.log file is the redirected stdout or stderr file. That file is not governed by log4j, instead it is just redirected output. There is no way to limit the size of that file. Your options are to
a) occasionally restart the JBoss AS service and between the stop and start, either delete or move the file.
or
b) Change your service init script to not redirect stdout and stderr. You don't really need them, any console log output already appears in server.log, or you could easily replace the console log with a rotating file logger.