Answer
A space admin exports each space, Space settings then Export, to XML for a clean re-import or to HTML or PDF to read. There is no bulk REST export, so it is one export per space.
What it holds
Internal wiki pages, specs and meeting notes, with their edit history.
Find the size
admin.atlassian.com, Site settings then Storage, shows Confluence storage in GB. It refreshes about every four hours.
What to enter on the form
Confluence in GB, and the year your company started using it. Seller form