Different styles have different editable areas. The one you have selected is narrower than others. It is designed to display as a narrow column down the center of the screen. There are many other styles with wider editable areas.
We can't assist you with changing the width of a particular style, as it is fixed. Our only suggestion is that you try a different style to see which one best suits your needs.
We suggest using one of the Responsive templates. These templates in conjunction with the Style Designer will allow you to adjust the width of the style. You can find these templates by going to Style > Change Templates and selecting Responsive.