Jeremy Herve on "[Plugin: Jetpack by WordPress.com] Thumbs size for Tops posts & pages widget"

As Stacy mentioned, you will need to add some custom CSS to your site. You can add this code at the bottom of your theme stylesheet, or in the Custom CSS editor available under Appearance > Edit CSS in your dashboard:

#widgets .widget_top-posts .widgets-list-layout-blavatar {
    max-width: 600px;
}

You will also need to change the image source to load bigger images. You can do so by adding the following code to your theme's functions.php file:

function jeherve_custom_thumb_size( $get_image_options ) {
        $get_image_options['avatar_size'] = 600;

        return $get_image_options;
}
add_filter( 'jetpack_top_posts_widget_image_options', 'jeherve_custom_thumb_size' );

Of course, you're welcome to replace "600" by another value! :)

Let me know if it helps.