Thumbnail image smaller in posts feed

Hi, I’m having an issue with the size of the image in the posts feed as most of them are showing smaller sizes, and filled only part of the frame size I defined. Please see the screenshot here(https://i58.tinypic.com/2e58su9.jpg). And, I’ve been changing the settings for thumbnail/featured image and cant seem to get it working. My site is at https://www.mminfozone.net(Excuse for the foreign language). I would be really grateful if someone could point me to which settings I need to change to get the consistent image size in the posts feed or what I might have done wrong.

Thanks.

https://www.ads-software.com/plugins/wp-rss-multi-importer/