Things also appear large if bring them home in a car, or assemble them inside a house or garage. They appear much smaller when they are outdoors, and especially when they have chickens in them.
I've seen a suggestion to make a mock-up from cardboard when planning a coop, to get an idea of how large it will really be. That probably works well for some of the small and medium sizes, but could be impractical for big shed-sized or barn-sized ones.
Having a coop already can make it easier to visualize, because you can plan it to be "the same size" or "this much bigger."