Change Width of Website

First of all this theme is well documented and most your questions are already asnwered here: https://colorawesomeness.com/themes/support/travelify/

Your featured images are too small in width to fit “full screen” consider replacing them with the recommended image sizes you will find on theme documentation,

To make theme itself look full-screen you can add this to Theme Options – Other – Custom CSS.

.wrapper {
	max-width: 1920px;
}
#branding {
	margin-top: 0;
}