Answer
Google Takeout for one account, or a Workspace admin uses the Data Export tool for the whole domain. Takeout splits into 50 GB files.
What it holds
Shared and personal documents, spreadsheets, slides and uploaded files.
Find the size
admin.google.com, Reports then Storage, gives per-user and total Drive usage.
What to enter on the form
Google Drive in GB, and the year your company started using it. Seller form