When I want to make a longer and/or wider quilt I add a strip of simple color blocks using the colors in the quilt across the top and bottom and/or sides depending on where I needed the added size, then add my borders. Sometimes I'll use up the scraps making 4-patches or pinwheels to make up the needed size.
I've also seen where someone has slashed the panel and made it into a attic window which will add size both directions. Another person cut the panel lengthwise and added a strip between each panel strip adding width. I have the fantasia tree of life panel and that's what I plan on doing to it but adjust the strips in a bargello fashion.