We've had a few questions about possible extra credit for Project 2. Here's a clarification that may (or may not) help you.
Loosely, what we are looking at is for you to explore the boundaries of what you can and cannot model with PDDL. Given that, something like just throwing a larger number of objects into a problem simply to make a bigger instance is probably not very interesting.
However, exploring the things you can do (for example) using the type hierarchies in the definition of the object types, defining different actions from the ones mentioned, extending the domain itself - these are all interesting things to do. These will of course lead to problem instances different from the ones that we have provided. Of course, these are only some suggestions.
Please do include a detailed explanation of whatever extra credit you choose to do (if at all) in your project report. We will try our best to provide you support if you run into problems while pursuing the extra credit, but in the interests of fairness we will try to help students who have questions about the mandatory portion of the project first.