Our API is based on our collective experience with CwC until this point as well as our interactions with Arrays in other languages. Some invariants that we agreed on for Array are mutability of contents and dynamic sizing. While there is certainly extra functionality that could be included, for example, some sort of iterative print function, we felt that this design was capable of being extended when necessary.
Fields necessarily constrain implementation. That's why we kept the fields as short as possible. The size and capacity of an array are necessary to ensure that dynamic sizing is possible. The data needs to be stored somewhere and since we are working with C, an array of Objects seems reasonable. While some other class of lists could be invented for the Array to use in its implementation, it's unlikely. Arrays are one of the most basic data structures in any language, so we felt it should rely on as little as possible.
The methods are also relatively concise. Getting and setting are standard and require indices. Pushback ensures that the user can add to the Array without knowing the length and so forces the implementation to allow that. Remove returns the object in the place of removal to enforce the user's maintenance of memory leaks. Finally the size method allows access to a private field.
These methods don't have much constraints other than that a custom destructor will be necessary in order to delete any objects that are stored in the array.