I think that this plugin should help you: wordpress.org/plugins/quick-featured-images. If you change the size of the featured pictures then go to the Appearance then go to the Editor then open the functions.php file and look for “thumb” and change it to whatever size you want it to be.

You can also check out these links where it showed even better: youtube.com/watch?v=mbn2Uqq53UQ and graphpaperpress.com/blog/change-featured-image-sizes-wordpress.