Small enough that it will actually happen

The most common way this goes wrong is picking something too big on a good afternoon, failing at it on a bad morning, and taking the failure as more evidence.

So make it embarrassingly small. If you are thinking about going for a run, the version that works is putting your shoes on and standing outside the front door.

And judge it by whether it happened, not by whether it felt good. It very often will not feel good, especially early. The point of the action is not to produce a nice feeling. The point is to put one thing back into a day that has had everything taken out of it.