Image sizes

Hi, The theme instructions specify Custom Header Width: 1920px Height: 1080px except I am finding that the image I prepared is getting cropped at the top and bottom. What size should it be? I am also a bit uncertain about the size for the featured content images on the home page. Somewhere I found 874x874 but this appears to be much larger than what appears on the page. Please advise. I haven't gotten to this yet but the theme instructions give Post Thumbnail Width: 1068px Height: 567px which is clearly not a thumbnail. I would appreciate it if you could give me all of the correct dimensions and perhaps update the instructions for everyone's sake! Thank you