Best images sizes for Featured Content boxes

I am a new Clean Box Pro user. I am trying to determine the best size/aspect ratio for a featured image that will display at the top of a post/page, and also in the Featured Content box, correct? In your doc, there are two difference sizes. We want all the images that display in the feature content boxes to be identical sizes/aspect ratio. Is there an option to crop them to a certain size? The temp URL for the site is http://37.60.237.241/~remierac/rcc/. Thank you.