How to reduce the thumbnail size on category archive pages

Hi,

I am sorry if this has been asked before. I am pretty bad at technical stuff and will appreciate a small help. I am using Solostream theme of ClearVideo for my blog. I notice that the category archive pages lists the posts under the category one after the other but before the post link, it displays the thumbnail in huge size. I want to reduce it to a small size or delete it. On the side bar there are links to posts which displays the small thumbnails along side. I want the thumbnails to be of same size in the main content area of archive pages when it lists the posts under the category. Just a small thumbnail at the side before the post link. But what I am getting is a huge thumbnail covering the whole width of the post above the link of each post.

Please check this page to see what the issue is: https://rajeshpiyer.com/category/social-media-marketing/

You can see the Recent posts links on the sidebar which has small thumbnails. Can’t I not have same size thumbnail on these archive pages when it is listing all posts under the category? Also, please let me know what should I do if I want to completely delete thumbnails on the archive pages before the listing of the post links?

Thanks in advance.

Rajesh