I think it would make a great center focus piece on a double bed. You'll have to decide if you want the rest of the quilt to be dark like the other colors or lighten it up. Also decide if you want the other fabric to be horse related or not. I would start with making some blocks, could be pieced blocks or just large squares, to add to the top and bottom to give it more length first before dealing with the width then work into expanding the over all size. Or you could decide on a just one fabric to add on to the length both ways so that the panel is centered and then the width, and add borders (drop down) from there. I can picture several ways to do this and would be fun. Making a scrappy D9P or scrappy yellow brick road pattern in the right colors for expanding the length both directions comes to mind also- would make it look more old fashion quilt design. The more I think of this the more ideas pop into my head. You'll have to maybe draw out a few ideas.