I'd be very strongly inclined to go with #4 and just say that anyone who actually cares about numbers that large should use 64-bit.
Uhm, 64-bit? What about large files (> 4 GiB) on 32-bit systems?(Say, if a range wants to provide random access to the bytes of an 8 GiB file?)
And what about large bit vectors? A 600 MiB bit vector needs > 2^32 bits for length...
