Getting out of your comfort zone is good, but taking on too much too soon can lead to abandoned projects or even worse, burnout.
This rule lets you grow and take risks, but not by too much.
Here it is:
Don't start a new project that is bigger than the combination of the two biggest projects you've completed.
The word "bigger" is generic and that is intentional. Size is quantified differently in different media. It could be number of pages, scope, length, etc.
At most, you'll be doubling the biggest thing you've made.
If you've finished a 100 page story, a 50 page story and a 75 page story, your next story should be no bigger than about 100 + 75 = 175 pages.
So I have to make bigger and bigger projects forever?
No. The rule only provides an upper bound on new projects. The lower bound is up to you.
What if I'm just starting out?
Think of the smallest project you could make. Start and finish it.
Then think of the smallest project you could make that is bigger than the first project. Start and finish it.
Now you have finished two projects, probably made some mistakes and also learned a lot. The rule can help you pick your third project.