What is the purpose of はある in this sentence?


> 顔の大きさの半分 **はある**
> であろう大きなマスク。([source](https://www3.nhk.or.jp/news/html/20180801/k10011558961000.html?utm_int=news-
> life_contents_list-items_014))

I've been told that this sentence is describing the size of the mask being
half the size of the face, Can't we simply say 「顔の大きさの半分であろう大きなマスク」 instead?




This type of は means "at least". That is, the mask covers roughly 55-70% of
the area of the face.

  * [Does は mean 'at least' in this sentence?](https://japanese.stackexchange.com/q/29438/5010)
  * [Can は be used to mean "at least"](https://japanese.stackexchange.com/q/33243/5010)

The sentence can be rephrased as 顔の大きさの半分はありそうな大きなマスク, 顔の大きさの半分以上あるであろう大きなマスク,
etc. But 顔の大きさの半分であろう大きなマスク does not have the same meaning.




This type of は means "at least". That is, the mask covers roughly 55-70% of
the area of the face.

  * [Does は mean 'at least' in this sentence?](https://japanese.stackexchange.com/q/29438/5010)
  * [Can は be used to mean "at least"](https://japanese.stackexchange.com/q/33243/5010)

The sentence can be rephrased as 顔の大きさの半分はありそうな大きなマスク, 顔の大きさの半分以上あるであろう大きなマスク,
etc. But 顔の大きさの半分であろう大きなマスク does not have the same meaning.

