message-send-mail-partially-limit

message-send-mail-partially-limit is a variable defined in `message.el'.
Its value is

nil

Documentation:
The limitation of messages sent as message/partial.
The lower bound of message size in characters, beyond which the message
should be sent in several parts. If it is nil, the size is unlimited.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.