Welcome to the board! Can you post a picture? That would make it easier to visualize border ideas. I have not seen a quilt with crochetting, but it sounds great. Is it purple and white? (my favorite color combination...)
Without a picture, a few ideas come to mind...maybe one of them might help? Does it really need to be larger? Maybe 7 x 9 is big enough? Lay the quilt out and "audition" white fabric as the border. Google quilts with crochetting and see what you find.
Obviously I can't really help, but maybe someone else here can. I have only been quilting 4 years, and I have a lot to learn yet.
Dina