Header image size?

[glymr]

Hello!  I would like to hold a contest to have people create a header image for this community: detroitbecomehuman.  However, I can't seem to find any resources to tell people what a standard header image size should be, ie "no wider than x number of pixels", "no taller than x number of pixels", or anything about whether they will degrade gracefully on mobile, etc.  Does anyone have any tips or links that might help?

Thank you very much!