This is attribute gives an estimate of the number of entries that
will be stored in the KeyMap. It is used to tune the internal
properties of the KeyMap for speed and efficiency. A larger value
will result in faster access at the expense of increased memory
requirements. However if the SizeGuess value is much larger than
the actual size of the KeyMap, then there will be little, if any,
speed gained by making the SizeGuess even larger. The default value
The value of this attribute can only be changed if the KeyMap is
empty. Its value can be set conveniently when creating the KeyMap.
An error will be reported if an attempt is made to set or clear the
attribute when the KeyMap contains any entries.