How big does my project have to be to unit test?

Related on HN: "Should you TDD a MVP?" http://news.ycombinator.com/item?id=4986665

I'd answer this question pretty much the same way I answered the MVP question: http://news.ycombinator.com/item?id=4986852

It's simple really:

For production code, test everything. anything less than 100% test coverage (outside of a one man team) is a disaster looking for a place to happen.

In my personal projects, I write no tests. And then I get to some point where _as I'm implementing_, I need to check it's working. Then I write my first test.

And so on, writing tests for anything that's awkward to verify as I'm authoring it, and also key points that I'm planning to refactor later (to ensure it still works afterwards).