Sketch a small chocolate bar and experiment with breaking it in different ways. Make sure you understand the claim and believe that it's true.
Does it matter which line you use to make the first break? The second break? Does the choice of break lines matter at all?
This problem doesn't involve trees, but you'll want to approach it in a similar way. That is, your induction variable will be the size of the object, and there are multiple objects of each size.
What's a good way to measure the size of a chocolate bar?
Your inductive step is going to involve splitting up the chocolate bar into two bars. Make sure that your definition of size has the property that the two new bars are smaller than the original bar, so that you can use the inductive hypothesis on the small bars.