An admin with the Manage Matters, Manage Searches and Manage Exports privileges signs in to vault.google.com, opens a matter, searches Chat, then clicks Export, enters a name, picks PST or mbox for the message content files, and clicks Export. Vault returns each matched message plus messages sent up to 12 hours before and after it in the same conversation, and writes a conversation as one document unless there is a 24 hour break, which starts a new document. Content files split into more than one zip above 1 GB for PST or 10 GB for mbox. Export files are available for 15 days after the export starts, then they are deleted and the export has to be run again.
What it holds
Direct messages, group messages, space messages with in line threads, and the files sent in them.
Find the size
The Admin console gives message counts rather than a byte figure: Menu then Reporting then User Reports then Apps usage lists Chat messages or reactions, Chat uploaded attachments, Chat spaces created and Chat conversations read on a 28 day window, and Download writes the chosen columns to Google Sheets or CSV up to 100,000 rows. Quote those message counts and the space count.
What to enter on the form
Google Chat in messages, and the year your company started using it. Seller form