Leave yourself a 2" - 3" on each side. If it is going to a LAQ it should be 3" - 4" on each side.
One time when I was making two 'matching' twin quilts for my 2 DGD's. I had purchased the backing on a road trip. When I got home ^&%*^%( I was only inches short. What I did was to add a small strip at one end of the quilt(s). Talked to my LAQ about my problem, she was able to line things up so after it was quilted I was able to unsew the strip and every thing looked fine.