Banner ads are created different.
In 2017, Google display network accept 18 different animated ad size. But is there some size more effective for some mistery reasons?
The answer is: YES.
Google does provide recommendations to ad size:
336x280 (Large rectangle, LREC)
300x250 (Inline rectangle)
These are the most effective sizes to go when the creative starts. These are all appliable to animated ads, HTML5 ads and also static image ads. Most advertising networks accept these size, so these size should be able to serve and start collecting data and start the optimization from there.
Of course, further optimization should be based on the real CTR data.
Image from demandamurray