Since the downstream calls to PyMem_Malloc and _PyLong_FromByteArray both accept size_t for their sizing, there isn't a problem there.

That said, I think the current limitation nicely protects us from harm.  If you were to run getrandbits(2**60) it would take a long time, eat all your memory, trigger swaps until your harddrive was full, and you wouldn't be able to break out of the tight loop with a keyboard interrupt.

Even with the current limit, the resultant int object is ridiculously big in a way that is awkward to manipulate after it is created (don't bother trying to print it, jsonify it, or doing any interesting math it).

Also, if a person wants a lot of bits, it is effortless to make repeated calls getrandbits() using the current API.  Doing so would likely improve their code and be a better design (consuming bits as generated rather than creating them all at once and extracting them later).

In short, just because we can do it, doesn't mean we should.