Your theme documentation will outline all exactly image sizes for every space on your blog. But, here is a general sizing guide:
Recommended image upload sizes for our themes:
@1x: 850px wide | @2x: @1700px wide
Home Page Featured Post sizes can be found here
with sidebar: @1x: 700px wide | @2x: 1400px wide
without sidebar: @1x: 900px wide | @2x: 1800px wide
@1x: 800px wide | @2x: 1600px wide
Curious about the difference between @1x and @2x sizing? You should definitely read this Help Desk article on Retina displays.