The names of the sidebars are a bit confusing. The sidebars labeled Top, Right, Left and Bottom all display floated on the right side of the screen (or left depending on chosen layout), the difference being their position and width. The Top and Bottom sidebar both span 100% of the available space, the difference being that one displays above and the other below the sidebar area. The other two, Left and Right, display between the Top and Bottom sidebar, the difference being that they both span 50% of the available space and float next to each other.
So in order to have both the Newsletter and Instagram widget span full width you need to move them to either the Top or Bottom sidebar.