NameNode heap size depends on many factors, such as the number of files, the number of
blocks, and the load on the system. The following table provides recommendations for
NameNode heap size configuration. These settings should work for typical Hadoop clusters
in which the number of blocks is very close to the number of files.
The NewSize is usually set between 1/6 and 1/8 to 1/10 of the HeapSize but it can be tuned.
It is well described in the following doc: