The ITEM_SIZE attribute
returns the size (in bytes) of an item, which includes the key and
data variables and some additional internal information. You can set
an estimate of how much memory the hash object is using with the ITEM_SIZE
and NUM_ITEMS attributes. The ITEM_SIZE attribute does not reflect
the initial overhead that the hash object requires, nor does it take
into account any necessary internal alignments. Therefore, the use
of ITEM_SIZE does not provide exact memory usage, but it does return
a good approximation.