Level of Detail
Think about the level of detail that it would be appropriate to build into the simulation. For example, do you need to include every part of every operation at each machine in the factory, or is it more appropriate to simply simulate the output of whole departments and how they interact with each other? Putting too much detail in a simulation means it costs more of your time to build and can also cloud the issues that the decision maker is interested in.
Think about how wide to make the simulation. Can you limit the boundaries of the simulation to the factory itself, or do you need to also include the systems that affect supply of raw materials to the factory?