Ah, I found another thread which seemed to answer my question. I followed the advice, which was:
You can use phpmyadmin and modify the table staticpage and change the column sp_content to a mediumtext.
I did it, went back and edited by staticpage, and same error:
You must at least fill in the Title and Content fields.
i.e. it seems to be dropping what I pasted in.
I tried doing it one line at a time and it really seems that it just can't handle the number of lines. But surely the change to mediumtext should've done the trick. I even checked in phpadmin again and the page has sp_content as mediumtext.