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).
I’d say the most annoying thing of long messages are
a) the need to scroll up to read the beginning of the message - sometimes not understood
b) reading through a thick block of text
So you could
Fwing the reader to a website can be a solution (i.e. for contractual stuff or for information) but any moment in which you send a user out of your chatbot means you are potentially letting that user go.
The next button sounds like a good idea, though your analytics will be less accurate if you do that.
Yes - don’t send really long messages. Break it up into smaller segments (not just for technical reasons) and for each message use common/familiar indicators that there are more parts. You can do that sequentially and cascade your messages
1/ message one
2/ message two
(Tweet storm format… people are familiar with this)
OR
message one…
message two…
(… indicates there is more. Use the … in the middle of sentence as to not indicate you’re at the end of a thought/sentence)
OR
message one, and…
message two, and…
(the and… indicates you’re in the middle of a thought)
OR
Just load all of your messages at once, and the user will scroll up to read through each.
In our experience scrolling is not the main problem 
with long messages.
People can scroll. The problem is with aestetics and perception.
TITLES
When people visit webpages they do not read everything. They scan titles and decide if that block of text worth reading. Do the same for messenger content.
BUBBLES
Devide your text into smaller chunks so the messenger will create separate bubbles for those blocks. Add 2-3 sec pause and typing… effect.
IMAGES
Separate large text blocks with images or stickers.
EMOTIONS
You can help read long test if you articulate main points with 
emoji. Even large blocks of text are easier to read 
if you create visual accents. This works well even for business 
chatbots like insurance or legal bots.
SIMPLIFY
Keep it short and simple. Write as if you were typing a text msg to your friend.
i would avoid long messages at all costs. it takes some effort to cut down the verbosity of a message, but it will improve engagement/retention if you do
Sometimes long messages are inevitable - especially in use cases where there are legal considerations. As @ArtemyMalkov says, this is a perfect use for emojis - pick ones that fit the tone of the copy so 
instead of 
if you’re an insurance brand.
All the option proposed here are good. I don’t think there is a right answer.
I think is better to use them all, based on the content of the text.
While not always ideal, we sometime link longer, more complex responses so that the users can view in a browser for easier reading.
That’s great advice! I found that it’s mostly how things work for WeChat. They design websites that look like something very native to the platform which is great for user experience!
From what we have seen no one reads long messages, having to scroll back up its really annoying and just straight up a bad experience. Also fb messenger caps messages at 320 characters and won’t even send more then that.
Whatever you are trying to get across to the user you should do it in way less characters. Think Twitter.