featured image smaller

i understand that the suggested size of a featured image is 910 x 607 pixels. however, my site is goign to need to use square images that are 300 x 300 pixels. I do not want them to be expanded out to fill up that space on full browser view when responsive resizes happen.

What do I need to change in charitable campaign pages so that the featured image will display at a maximum size of 300 x 300?

thanks!

https://wordpress.org/plugins/charitable/