Just find a pattern for the largest size quilt you can. Then add extra borders to increase the size. Even if some of the borders are larger on the ends than the sides. or you can add an extra border stripe on ends as your first border and not put any of that color border on the sides. I have also learned to wait to purchase my border fabrics until the main part of quilt is done. You can often add totally different fabrics that coordinate with the center of the quilt. I am sure you will get lots of help from board members. Good luck and enjoy the process.
Kat