Hi barefootbutterfly, There is a way to do this but it will be much easier once we introduce the Collage Dropdown to fix the ratio to “Facebook Cover” which is coming in a future version.
For right now, the key is to watch the dimensions of the collage image which are down the bottom right of the screen next to “Zoom”. A Facebook Cover image needs to be as close as possible to 379×1024 (which will be reduced to 315×851 when you make it a Facebook cover photo). So, you can choose any layout and then adjust the spacing and proportions (or even rows and columns) until it is as close to 379×1024 as possible. The “Grid” collage with 3 horizontal spaces is probably the closest to what you’re after.
The size change to Gooify is of high priority and will also be in a future Ribbet version.