In the PyPI top 5000 projects, I found 32 projects using "Py_SIZE(obj) =
new_size": 8 of them are written manually, 24 use Cython.

8 projects using "Py_SIZE(obj) = new_size":

* guppy3-3.1.2: src/sets/bitset.c and src/sets/nodeset.c
* mypy-0.910: list_resize() in mypyc/lib-rt/pythonsupport.h
* pickle5-0.0.12: pickle5/_pickle.c
* python-snappy-0.6.0: maybe_resize() in snappy/snappymodule.cc
* recordclass-0.16.3: lib/recordclass/_dataobject.c + code generated by Cython
* scipy-1.7.3: scipy/_lib/boost/boost/python/object/make_instance.hpp
* zodbpickle-2.2.0: src/zodbpickle/_pickle_33.c
* zstd-1.5.0.2: src/python-zstd.c

24 projects using "Py_SIZE(obj) = new_size" generated by an outdated Cython:

* Naked-0.1.31
* Shapely-1.8.0
* dedupe-hcluster-0.3.8
* fastdtw-0.3.4
* fuzzyset-0.0.19
* gluonnlp-0.10.0
* hdbscan-0.8.27
* jenkspy-0.2.0
* lightfm-1.16
* neobolt-1.7.17
* orderedset-2.0.3
* ptvsd-4.3.2
* py_spy-0.3.11
* pyemd-0.5.1
* pyhacrf-datamade-0.2.5
* pyjq-2.5.2
* pypcap-1.2.3
* python-crfsuite-0.9.7
* reedsolo-1.5.4
* tables-3.6.1
* thriftpy-0.3.9
* thriftrw-1.8.1
* tinycss-0.4
* triangle-20200424