Hello,

I saw the screenshot you attached.

The blog thumbnails are controlled by wordpress itself; you can set the thumbnails sizes from Settings >> Media.

Screenshot for Clarification: https://ibb.co/4PvpFTJ

You will also need to regenerate thumbnails using the free plugin.

https://wordpress.org/plugins/regenerate-thumbnails/

Please take a complete backup of the website before regenerating the thumbnails.

Regards.
Xtemos Studios