The theoretical limit is 2^64 bytes (16 exbibytes, 18.4 exabytes).
However, as of now, the practical limit is much lower, because multipart upload is not yet available. You can’t upload an object one part at a time- the whole thing has to be uploaded in one session without losing contact with the satellite. With the internet the way it is, it might be difficult to get as far as a single terabyte.