Featured images are different sizes

Hi,

I’ve got the theme set up to display the first post in full and subsequent posts as excerpts on the home page but the featured images are all coming out at different sizes and it looks awful. I’ve searched through previous posts but there doesn’t seem to be anything that exactly matches this issue. I’m not sure whether this is to do with the size of the original images or if it’s something I could change in the CSS? I’m using a child theme.
This is the site: https://www.lifeonveg.com
Any help would be much appreciated.

Thanks,
Amy