There is no indication as to the the size and format that a video attachment needs to be when being sent through Facebook Messenger, at least in the documentation here: https://developers.facebook.com/docs/messenger-platform/send-api-reference/video-attachment.
Is there a limit on size? What format does it need to be in?
I was looking for this information too. I decided to try to upload files of different size to see what happen. When you try to upload files bigger than 25MB, Facebook Messenger display a error message stating that 25MB is the limit (hey!).
Note that I tested that on a generic file, not specifically on a video.
It says directly in the link posted by the OP:
Sending Attachments The Messenger Platform allows you to attach assets to messages, including audio, video, images, and files. Max attachment size is 25MB. There are three ways to attach an asset to a message: