Your theme documentation will outline all exactly image sizes for every space on your blog. But, here is a general sizing guide:
A standard size for all images across your blog posts and featured images is 1560px in width. For featured images, in addition to being 1560px in width, the image should be a minimum of 1280px in height (height can be variable for an image beyond this, but this is the min height). At this size, images will have no problem fitting into featured image thumbnails, and will look good within blog posts, regardless of whether your posts feature the sidebar, or display in the “full-width” layout.
Landscape Featured Post Size
Note that the 1560px size assumes you’ll always have two featured posts featured at the top of the Home Page (like this). If you ever switch to only one featured post (like in this example, with the perfume post at the top), the featured image for that single featured post would need to be uploaded at 2080x1280px.
Curious about the difference between @1x and @2x sizing? You should definitely read this Help Desk article on Retina displays.