Jue,
Yes, for better look images should have a equal sizes, as per sample that I sent.
You can set add size here: “Visual Adverts” –> “Settings” –> “Advert Image Size”
But after this, please use “Regenerate Thumbnails” plugin (https://www.ads-software.com/plugins/regenerate-thumbnails/) or similar one for properly displaying of custom thumbnails after customization of this plugin option.
But, I want to pay your attention on that fact that if your images is smaller then size that you set, image will be displayed in as it is, because it is default WordPress feature.