Size of raw response in bytes

Just take the len() of the content of the response:

>>> response = requests.get('https://github.com/')
>>> len(response.content)
51671

If you want to keep the streaming, for instance if the content is (too) large you can iterate over chunks of the data and sum their sizes:

>>> with requests.get('https://github.com/', stream=True) as response:
...     size = sum(len(chunk) for chunk in response.iter_content(8196))
>>> size
51671