As we don't currently have banner art I've decided to go to the community. The community decides the winner by upvoting the banners they want to see. It doesn't matter if you're amazing at art or not we welcome all artists. Now get out there and create your art! It will end in a week.
What should be the size/resolution?
I went to another communities Banner competition, and all the submissions seem to be 730 x 280 pixels. I couldn't find anything in the documentation about banner sizes.
From what I've tested most banner sizes work I just tried 4k and it works perfectly
From my testing every resolution works so whatever feels the best
Whatever wins, just be sure to scale it down to something suitable. I've seen mobile users particularly complaining that some community banners are stupidly large files that eat through data.