Reading Re^10: XS.c: loadable library and perl binaries are mismatched (got handshake key 0xc100000, needed 0xc180000) I was encouraged to confirm what rgren925 reported. I constructed a reply with about 120kB in it and used the Preview button to check it. All of it was rendered and there was no warning (that I could see) that it was over the size limit for posting. When it was subsequently submitted it still rendered fine and it was only on a further navigation to the same node that it did truncate to ~64kB as I expected.
This is not great from a UX perspective. The preview should either warn about the size or truncate it like the final submission would do or (ideally IMHO) both. The full text could still be retained in the textarea for ease of editing.