In an elevation view of the storefront you need to select the gridlines one at a time (tab thru until you've highlighted one, hiding the mullions may make this easier). With the gridline selected the "add/remove segments" button will be active up on the toolbar. Click on the segment(s) that you want to remove in order to creat your larger, single panel. Note, the gridlines will "remember" the omitted sections; you can add them back at any time later on if need be.