Photo best practices

What does changing the size of the hero image to 480 to 895 to 1152 x 2560 do? I had thought it would make the height of the header less tall, but it doesn’t do that. Does it just make the image size smaller/potentially more pixelated? If so, is there any way to make the size of the header smaller? I just switched to Renaissance from Discover and the blog header is really big, to the point where the content is below the fold on a 13" desktop.