The top of the Miramar Home Page allows you to feature 1 or 2 posts. Read how to turn this feature on and edit the featured posts here.
Below is a guide that shows you best practices for any featured images attached to posts in this top section.
With both Featured Posts enabled
When you have both featured posts enabled, the dimensions are 700 x 490 for Post #1, and 450 x 490 for Post #2. Double those values if you have Retina images enabled!
With only Post #1 enabled
If you are using only the large, landscape featured image (i.e., you have disabled Post #2), try sizing your featured image for Post #1 at at 1550 x 630px. This should be large enough to fit the space at the top of the Home Page, but also tall enough to look good within your Home Page blog feed. You can double this value for Retina displays if you wish, but be sure to save the image at lower JPG quality to reduce overall file size.