@franktan: I see that you are using Catch Box Free theme, where the recommended featured image size in Width: 644px and Height: 320px. See the theme instructions page at http://catchthemes.com/theme-instructions/catch-box/. This slider in Feature Post Slider with Content width.
But if you upgrade to Pro version, you will get option to make the slider full width of your site and also get option to choose from “Featured Image slider, Featured Page slider, Featured Post slider, Featured Category slider. In Full Width slider, the recommended size is Width: 980px and Height: 400px. But if you use Featured Image Slider, height can be any you like. For more see the theme instructions of Catch Box Pro theme at http://catchthemes.com/theme-instructions/catch-box-pro/
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