Slider image size vs Post Image Size

I am aware that the slider image on my home needs to be 720 x 318px and I am happy with that, however I would like my Post Image Size to be 221 x 221px and not the same size as the slider image.

How do I change my Post Image Size to be different to my Slider Image Size?

Here is the link to our website: http://www.postmatric.co.za/
Currently my post image size is exactly the size and ratio I would like it, but using the same image in my slider obviously doesn’t work as it leaves a lot of empty space on the right.