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This graph shows that online/mobile ads that only take up a small proportion of the screen are not looked at for long.

Video ads work well if they occupy between 20% and 50% of the screen (perhaps any more than this and the ad gets in the way of what of viewer is trying to read/see?)

Static display ads work particularly well if they occupy at least 1/3 of the screen.

For more details, see this paper produced by Dentsu.