How to set post featured image size?

Try with the given below code example,in this example defined thumbnail size is 150px wide and 150px high. You need to place in function.php file.

if( function_exists( 'add_theme_support' ) ){
        add_theme_support( 'post-thumbnails' );
        set_post_thumbnail_size( 150, 150 ); // default Post Thumbnail dimensions
}