The max size of each backup file is 2MB.
log4net allows the main log file to grow up to 2MB, then renames it as the backup file, and starts with an empty main log file. It will continue doing so until there are 10 backup files. When there are 10 backup file, and the main log file needs to be rolled over, the oldest backup file is deleted.
http://logging.apache.org/log4net/release/sdk/log4net.Appender.RollingFileAppender.MaxFileSize.html