code: func NewSizeLogger(fileDir, fileName, prefix string, fileCount int, fileSize int64, unit UNIT,
  	logScan int64, logSeq int) *FileLogger {
  	sizeLogger := &FileLogger{
  		splitType:  SplitType_Size,
  		mu:         new(sync.RWMutex),
  		fileDir:    fileDir,
  		fileName:   fileName,
  		fileCount:  fileCount,
  		fileSize:   fileSize * int64(unit),
  		prefix:     prefix,
  		logScan:    logScan,
  		logChan:    make(chan string, logSeq),
  		logLevel:   DEFAULT_LOG_LEVEL,
  		logConsole: false,
  	}
  
  	sizeLogger.initLogger()
  
  	return sizeLogger
  }
docs: // NewSizeLogger return a logger split by fileSize
  // Parameters:
  // 		file directory
  // 		file name
  // 		log's prefix
  // 		fileCount holds maxCount of bak file
  //		fileSize holds each of bak file's size
  // 		unit stands for kb, mb, gb, tb
  //		logScan after a logScan time will check fileLogger isMustSplit, default is 300s