All I can say is that camera placement is important when determining scale.
Ok, I have more. Objects that are distant tend to be sort of clouded by atmospheric haze. Those objects also appear less saturated than near objects. Another visual cue that helps us to determine size is occlusion. If there is a familiar object, such as a tree, between the viewer and the airship, that object gives the viewer some sense of scale. If the tree is big, but looks small next to the airship that is behind it, the airship must be huge. Many psychology books detail human perception. I recomend finding a good one.
good luck
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