private long cellBeforeFlushSize() {
    // make one cell
    byte[] row = Bytes.toBytes("A");
    byte[] val = Bytes.toBytes("A" + 0);
    KeyValue kv = new KeyValue(row, Bytes.toBytes("testfamily"), Bytes.toBytes("testqualifier"),
      EnvironmentEdgeManager.currentTime(), val);
    return ClassSize.align(
      ClassSize.CONCURRENT_SKIPLISTMAP_ENTRY + KeyValue.FIXED_OVERHEAD + kv.getSerializedSize());
  }
Test big cell size after in memory compaction. (HBASE-26467)
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestCompactingToCellFlatMapMemStore.java