Featured Post with different image size

I’m trying to figure out how to work with 2 different size images per post. I have a featured post, the latest post, which has a larger image. But when it’s not a featured post I’d like to use a smaller size image. To add to that when you click on any post I’d like the post to show the larger image as well.

I see a lot of css galleries do this, but not sure how. Any help?