Does your proposed solution work properly when docstrings are turned off?  Try undefining WITH_DOC_STRINGS.  I bet you need to make the size 1 in that case.

Also, the length must be len(f.docstring) + 1, to account for the trailing \0.

Also, error C2133 doesn't look like it's applicable.  On line 71 it's not being stored in a structure.  Dumb MSVC.

Also PyDoc_SIZEDVAR is a bad name.  PyDoc_SIZED_VAR would be an improvement, but I think PyDoc_VAR_WITH_SIZE is the name you want there.

And, I'm still not a +1 on this approach.  Can you propose something else?

Finally, I will note that the "buffer" preset would work just fine in this file, if you dumped the buffer just above the PyMethodDef array.