How do I change default Image sizes in Blog listings?

Hello, This is a question about a Blog page using this theme. This is with regards to the Blog page that shows the ABBREVIATED introductions to each blog-post, not the full versions of the pages. If I've selected a Featured Image it shows an image also with these abbreviated intro's to the posts. My questions is: How do I change the default (not just for individual images but across the board) of how large those Featured Images appear in the Blog posts-listings page? I'm not referring to the images on an individual post; I'm referring only to the automatically-generated images of the abbreviated post-descriptions. I want to have ALL of those introductory images changed to smaller, as well as any future images as well. **Ideally it would just be changing the default length of the Images and keeping the Width the same (all the way across). How do I do this? Thank you for your help.