It says it on when the upload page comes up at the bottom corner that 2560x1440 is recommended because it's used on TVs and some people have High res tvs.
If you go to the channel art guidelines support page it'll give you the resolution as 2560x1440 you can also download a template for the banners which will let you know which areas will be visible on which devices.
It's not the clearest image to be fair so it might be worth just Googling "One Channel Banner Template"
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