How do I control the size of the featured image for blog posts?

Hello!

What is the best way to control the size of the featured image of a blog post? Currently, when I post, the featured image shows up as a FULL size so it is very difficult to see the list of blogs underneath.

I tried looking at the CSS style setting but I dont see any options for the feature image (or maybe I missed it?)

Thank you very much for your guidance!
Calvin