Change size of featured image on Elated Kreate

Please help me. I use the Elated Kreate Theme.

I want to make my blog post page have an image size like the one in the center.

https://drive.google.com/open?id=0B-lEd9FwmW3gVGJvMGxUcm5acjQ
I used this .post-thumbnail img { width: 300px !important; height: auto; }

and also edited the image with direct url with inspector to make you see what I want.

I want the feature image to have a size of 735×1102, then if the width is 300px it can have an auto height.

I love the way related post is showing. Something like that. https://drive.google.com/open?id=0B-lEd9FwmW3ganV6VThSSVBDRUU
I have read a lot of posts on changing the feature image on wp themes. But I’m did not get them.

Please Help!! Thanks