I have a torrent with 166 video files (mp4) and one folder containing images (jpg). I downloaded the entire torrent last March without any issues and I've been seeding them ever since. Today, I decided to assign "ratings" to quite a dozen videos. This somehow created a mismatch between the filesize expected by Tixati and the actual size of the torrent. This caused the seeding of this torrent to stop. Is there a way I can instruct Tixati to ignore this trivial change in file size? (I cannot imagine it being more than a byte).

You can't, it's against the purpose of the protocol, integrity.
And for other reasons.

No, the files must not be changed at all to be seeded, this would be a limitation in many if not all torrent clients.

OK, I removed the ratings, but I still cannot seem to seed the files. Is there anything I can do to seed them without resorting to downloading them all over again?