Define featured image size

I have a question related to the posts page
On that page :

How to define featured image size? eg I want them to be 800x400px

Thank you in advance!


You can use this CSS code to set the default image size:

.blog img.wp-post-image {
    width: 800px;
    height: 400px;

But, I think for the more best experience in displaying the images, you need to provide the image that has size 800x400 too.