I have yet to make a quilt larger than crib size where I didn't find a difference of 1/4"-3/8" from one long side to the other. By measuring thru the middle of the quilt (as advised above), cutting the borders to that measurement, and then easing them onto the quilt, each one turned out beautifully.
FWIW, even tho I'm listed as a "senior member" of the QB, I'm only an advanced beginner, at best. I was lucky enough to have found the QB before I made my first quilt. What a blessing it has been to learn from people who know so much more than I do! Good luck!