Decreasing the size thumnail image

Do you mean you want to change the image size that is loaded and displayed within the Hueman Posts widget?

Using a Child Theme, you can override the image sizes the theme creates by adding your own function to the child theme functions.php file, like so:

function custom_image_sizes() {
    add_image_size( 'thumb-medium', 280, 158, true );
  }
add_action( 'after_setup_theme', 'custom_image_sizes' );

The thumb-medium is the image used by this widget, its default dimensions are 520*245px.

Hope this helps.

yes i want to little bit decrease the image size in the primary and secondary bar.So should i apply above CSS in custom CSS?

No, the code I gave you is PHP and should be placed within your child theme functions.php file.

You will also need to regenerate your thumbnails which simply forces the theme to regenerate all images again using the correct dimensions.

Hope this helps.