Object Size
The size of objects on the screen is a function of context—to understand the size of an object, the object has to have a referent. Referents can be anything in the visual field that people can use to judge size.
Tractors example. How big is a tractor?
Machine seat example How big was the "machine seat" built in the movie Contact?
Image size
Image size is contextual. An important point about image size is size constancy. For example, we know that when we see a huge close-up of a person on the movie screen that the person is not a giant. But we can create a giant by using a visual referent.
In terms of screen size—the amount of screen real estate you have to work with—larger screens can be more impactful. The large screen lends itself to visual effects and the use of landscape as an important visual element while the small screen tends to favor interactions among people.
How does this translate from film to other media? Screen space in the web, for example, can be controlled by the way we divide and fill it. Be conscious of the size of elements that you place in the visual field and the purpose the size serves.