Best image size for theme Dev of Mike Oliver

Hello I want to know how to find of exactly image size of theme Dev of Mike Oliver to setting -> media and auto crop with right image size. Can help me about that? I need it to add something like this:
// Adds image sizes.
add_image_size( 'sidebar-featured-thumb', 70, 60, true );
add_image_size( 'featured-image', 800, 400, true );
add_image_size( 'header-hero', 1600, 800, true );
It will good for my website. Can you give me detail how to know exactly that size for this theme?