Need to customize featured image in the 'posts' block.

I would like to use the featured image in the posts block so that I can show an image there if I want to. Currently the featured image just displays it full size above the title. How can I get it to take up the space that I highlight in the picture? Also, is there a way that I can get the text to reman the full size if there is no featured image?

Another question, how can I change the boldness of the text that is the title on my blogposts? Changing the CSS seems to not do anything for me.