Answer
Export lists and reports to CSV, and use the QuickBooks Online export or a backup through Intuit's own backup service for a full copy.
What it holds
The ledger, invoices, bills and payments.
Find the size
There is no storage readout. Quote the number of transactions or the years of books instead.
What to enter on the form
QuickBooks Online in records, and the year your company started using it. Seller form