Set size of excerpt image on blog

Hi, I really love this theme but am having a hard time with the images that display on the blog post excerpts. I want them to be a specific size and cropped to that size, rather than keeping the proportions of the image in the post (it looks very sloppy on my blog page otherwise). But I cannot find where I can change this.

I have a custom CSS plugin, if there is specific code I can add there, could you please give it to me? (CSS is not my forte)

http://autumns-garden.com