Historically, there has been critically little documentation for Memento, and this is why I, as a user, developed the wiki based on what I had learned, often by trying things and seeing what happened. I never tried to fill up a text field or a rich text field to see what happens and at what point it happens. It would not be very hard to do with a script. Maybe I'll do it some time.
The sizes of many things, including field values, can be found in JavaScript documentation. The number for JavaScript strings, which hold text field values, appears to be 2**53 - 1 (2 to the 53rd power minus 1). You could try the JavaScript docs...