Hi Samantha, add the following below your existing custom CSS. To widen the content area so the featured posts can be wider, add the same amount to the width and max-width below. Example, widen #page to 1260px (+300) and then widen max-width in #content to 860px (+300).
In the CSS edit page, put into the "content width" field below the CSS edit area, the same number you use for max-width.
Remember that if your featured image is not 560px wide, the theme will not enlarge it, it will use it as is, such as your "nino" image, which is only 360px wide. Also, with white backgrounds on the images, they may look like they are narrower than they are. For example, the boots image is showing as wide as it can for the standard width theme (560px) but because of all the white space around the boots, it looks smaller (it is actually a 1571px wide image you uploaded). If you want the boots to be a lot larger, you will have to edit the existing image and crop out some of the surrounding white space.