The Py_SIZE() macro is not documented.  It should be.  It is very useful along with PyList_New(positive_number) after using PyList_SET_ITEM() to fill in up to the first positive_number elements of a list object in the most optimal manner by avoiding numerous redundant array resizes and error checks along the away.

The Py_SIZE() macro was introduced (in 2.6 I believe) and is specifically intended for use as an lvalue (see http://bugs.python.org/issue1724 to confirm that).  It currently has uses in several places in CPython's core and modules as well as within some third party extension modules and tools such as Cython).