The payload seems to be limited to 3000 characters by the Stack Exchange engine itself:
**However,** [the actual field returned by the API may be longer][2], as the user's 3000, markdown characters are expanded into HTML.
As stated in "Can we have maximum length of string in API help pages?", the API itself enforces no length limits on the data and...
we're not giving hard guarantees as to the maximum length of strings returned.
That question introduced the suggested_buffer_size field in the self-documentation, but (A) that was just a suggestion and (B) the suggested_buffer_size field seems to have been dropped from the current version of the API.
So, there are no length limits published, and length is limited by the underlying SE engine, not by the API.