BlueSky Banner Sizes
The internet says they should be 2048 x 2048. I have no idea where the internet gets this from (perhaps an earlier version of BlueSky?), but a day of experimentation and screenshots suggests that the optimal size is 2048 x 1024px:
To see how much of a banner will be visible on various devices:
Photoshop PSD template for making BlueSky banners:
And the .AI files with the artwork from this page and the checker tool: