What's the meaning of "of any size"?
The crater — the first of any size found under the Greenland ice sheet — is one of the 25 largest impact craters on Earth

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It means “no matter what size”, or “not even of different size”.

In this sentence it means no craters are found before this. No small craters, no large craters, no craters at all were found before this one.

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the first of its king - regardless of its size - meaning extremely large or really small of this specific type.

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A large or big size (not small). Admittedly, it is illogical.

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