@brexous: I see that you are using Catch Everest Free theme, you can see in theme instructions page http://catchthemes.com/theme-instructions/catch-everest/, the slider take the featured image size of Width: 1140px and Height: 450px and the Content featured image as Width: 690px and Height: 462px. Sorry there is no option to change that. You might want to try plugins such as Thumbnail Editor to edit those thumbnail.
Or simply upgrade to Catch Everest Pro theme, where you have option to choose image size in the Post and Page “Catch Everest Options” meta box. Also you have option to use independent Featured Image Slider to add independent slider images.
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