Based on observations, I can say that there is no fixed size. I log in to IF from 3 different devices - tablet, laptop and mobile. And the Siggie box changes in all 3.
From my tablet, I've seen 16:9 size ratio siggies filling up the box perfectly while other ratios require scrolling. But from mobile, even larger ratios like 3:2 are appearing without scrolling.
Not only Siggie, the Forum Banners also appear variably in different devices. And no, I'm not talking about the dimensions that have been specified where it says that middle portion is visible in mobile. Because from my mobile, I see slightly more than the middle cut off, but not entire banner.
There is a lot of variation in Graphics related things depending on which device you log in from đ.
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