We need to install this plugin on our WordPress: https://wordpress.org/plugins/simple-image-sizes/ 

After installation active the plugin and go to the settings:

Click add a new size of thumbnail:

Click Validate and then save changes.

Go to Listivo Panel > Design > Listivo Ad Card and set it here.

Save changes and go back to the plugin settings and scroll to the bottom and hit button: 

When the plugin it’s finished working you have all images on your page with the new size.