An owner on the source organization generates a token with the repo and admin:org scopes, then POSTs to https://api.github.com/orgs/ORG_NAME/migrations with the repository list and lock_repositories true. Poll the migration status endpoint until the state reads exported, then download the export as a .tar.gz from the migration's download URL. The export file must be under 20 GB, exclude_attachments true trims it, and GitHub deletes the file after seven days.
What it holds
Git repository history, issues, pull requests, comments, releases and organization membership.
Find the size
The docs name no per repository size readout in the web interface. Quote the export file size against GitHub's stated limits: 10 GB recommended on disk for the .git folder, 2 GB enforced per push, 100 MB enforced per object, 5,000 branches.
What to enter on the form
GitHub in GB, and the year your company started using it. Seller form