The amount of collaborators is really the only limitation here. Free GitHub users now get unlimited private projects with up to three collaborators and for best performance, github recommends to keep any repo whether free or paid under 1GB each.

I was happy to read that they were offering free private repos, however, I'm leaving Github for good next week (except for public portfolio and archived work.)

I go to make my public repo private and get this crap, smacking me in the face:

• You will permanently lose:
  • All stars and watchers of this repository.
  • All pages published from this repository.

• You can upgrade your plan to also avoid losing access to:
  • Codeowners functionality.
  • Any existing wikis.
  • Pulse, Contributors, Community, Traffic, Commits, Code Frequency and Network on the Insights page.
  • Unlimited collaborators. This repository will now be limited to 3 collaborators.
  • Branch protection rules.

So, basically it's a sales maneuver.

Use Bitbucket, which was the first one to offer free, private repos for the sake of supporting developers - which also allows for unlimited hooks, presents it's own board for each repo JIRA/Trello-style, a wiki Confluence-style, and up to five contributors on any free account. It can also be integrated with Slack at no cost.

Maximum 3 collaborators. Also free private repo is only for personal/individual projects, not for organizations

Officially, the only limitation is to have up to three collaborators per repository. Size of the project is irrelevant.