What is used to represent an object that is too large to be built at actual size?

You could call something like that a model or a scaled down model or a set or even a blueprint, depending on what you are doing. You use different things to capture ideas for movies or play, and even when you think you might be able to build something really huge, you draw it up and build scale models to test it beforehand... so you don't mess it up in the huge scale of reality.