public long getRegionSize(byte[] regionId) {
    Long size = sizeMap.get(regionId);
    if (size == null) {
      LOG.debug("Unknown region:" + Arrays.toString(regionId));
      return 0;
    } else {
      return size;
    }
  }
Returns size of given region in bytes. Returns 0 if region was not found.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/RegionSizeCalculator.java