I am a complete newby to coding so take this with caution but I figured out a hack. Go into your functions.php and find the image sizes section. There you can change the hieght of the archive thumbnail to whatever you want it to be. I don’t think it will automatically change the archive thumbnail size of previously uploaded photos so you will have to go back and completely delete the old featured image and reload them to your site. It should work though for any new posts. I hope this helps.
