How to change image intrinsic size ?

Hello, I would like to change the image intrinsic size, for exemple on the home page, it’s 400×200, the size display well but the image doesn’t look good, kinda blurry.

I did this in function.php, line 29 :
add_image_size( 'post-image-thumb', 512, 512, true ); // 400, 200

But it doesn’t seem to work, how can I do this ?