Answer
A workspace owner runs Tools then Import/Export data. Public channels export on any plan; private channels and DMs need a paid plan and a compliance export.
What it holds
Channel messages, direct messages and shared files.
Find the size
There is no size readout in Slack. The export zip is the measurement, so quote it after the export or give a message count.
What to enter on the form
Slack in messages, and the year your company started using it. Seller form