I would assume sending several short messages is better than one big long one. Some services pose restrictions on the message length. In Messenger Platform, for example, there’s a limit of 320 characters. What are some good practices, then, when you need to send long message?
I’ve seen bots that provide a “next” button that let the user to read on, one short message at a time; I’ve also seen bots that give links to the entire message(often times articles).