500 x 1200 is the size to fit the home slider. But in other themes you might be sorry about having such skinny / tiny 500px featured images. So you should go taller than 500. But for the Simply Read theme, if you go 1200 your full width will appear on the home slider (cropped vertically to 500). If you go narrower, it will stretch out what you’ve got. If you go wider (more pixels, like 1920 or 2560) it will crop in, often resulting in confusing pix. Of course for some taller images, just taking 500px out of the center won’t be great either, but it’s the best you can do.