More information on how to use blog template please see theme documentation.
To make changes update safe you will have to use a child theme for that. A sample Travelify Child Theme can be downloaded from here.
Afterwards you can use this function via Child Theme functions.php
function travelify_child_theme_image_size() {
add_image_size( 'featured', 670, 670, true );
add_image_size( 'slider', 1018, 460, true );
}
add_action( 'after_setup_theme', 'travelify_child_theme_image_size', 11 );
Again image sizes are just an examples and you can use any size you want.