private long getRegionSizeMB(RegionInfo hri) {
    ServerName sn =
      masterServices.getAssignmentManager().getRegionStates().getRegionServerOfRegion(hri);
    if (sn == null) {
      LOG.debug("{} region was not found on any Server", hri.getRegionNameAsString());
      return -1;
    }
    ServerMetrics serverMetrics = masterServices.getServerManager().getLoad(sn);
    if (serverMetrics == null) {
      LOG.debug("server {} was not found in ServerManager", sn.getServerName());
      return -1;
    }
    RegionMetrics regionLoad = serverMetrics.getRegionMetrics().get(hri.getRegionName());
    if (regionLoad == null) {
      LOG.debug("{} was not found in RegionsLoad", hri.getRegionNameAsString());
      return -1;
    }
    return (long) regionLoad.getStoreFileSize().get(Size.Unit.MEGABYTE);
  }
Returns size of region in MB and if region is not found than -1
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/SimpleRegionNormalizer.java