What size are my blog post images? (Miramar)

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:

Miramar: 
@1x: 850px wide  |  @2x: @1700px wide
Home Page Featured Post sizes can be found here

Brevier:
with sidebar: @1x: 700px wide  | @2x: 1400px wide
without sidebar: @1x: 900px wide | @2x: 1800px wide

Archer:
@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.