Change Size Featured Image

I would really like to resize my featured images. At this moment they are not high enough. I would like them to be as big as the photo that I choose for every post. Now it’s a standard size and it crops the photo.
Can someone please explain how I can do this?
I’m new to all this (yes, a real newbie!), so if anyone could explain it so I could understand it: that would be really amazing!
Thanks!