How can I change the default featured image size in the blog post?
You have to open the theme’s functions/general.php file and on line 182 you can modify the default image size:
add_image_size('post_box_img', 580, 250, true);
the number 580 refers to the width and the number 250 refers to the height of the image.
Important: Please note that after you do this modification, you would have to reupload the images, as WordPress crops the images to the predefined sizes during upload. If you have lots of posts, another option would be to use the Regenerate Thumbnails plugin that will automatically resize all the currently uploaded images.