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Right, this one: "a simple shadow of the function as a
module level function".  If it affects all threads (which it
does), then a module function is a natural place for it.  If
I a saw a method on the Thread class, the most natural (to
me ;-)) assumption is that a_thread.stack_size(N) would set
the stack size for the specific thread `a_thread`, but not
affect other threads.  Part of what makes that "the most
natural" assumption is that Thread has no class or static
methods today.  As a module-level function, no such
confusion is sanely possible.

Sticking "stack_size" in threading.__all__, and adding

from thread import stack_size

to threading.py is all I'm looking for here.  Well, plus
docs and a test case ;-)