Using the Iterator
You need to dereference the iterator to get the item it points to.
So what you need should be as simple as
it1->findLargestBox()
Notice the -> instead of .
List or Vector?
You can just use the existing C++ implementation: std::list
Also, are you sure you need a linked list and not a dynamic array (std::vector)?
What are the properties that you need for this container to have? (What makes the linked list the best solution for your needs?)