Answer
A site admin runs a backup from admin.atlassian.com, System then Backup manager, with attachments included. It produces one zip.
What it holds
Issues, tickets and their comment threads, with who changed what and when.
Find the size
The same Storage page at admin.atlassian.com breaks the figure out by product.
What to enter on the form
Jira in records, and the year your company started using it. Seller form