I cut just about everything on a table that is 18" x 24". As long as you fold the fabric so the folds are parallel with the selveges, and cut on a 90° angle, your fabric will be straight.
I fold the fabric selvege to selvege (the long way), then I fold it again. At this point the yardage is about 9" wide and 4 layers deep. I square off the raw edges at the end - line the ruler up with the selvege end and the folded end, and cut the raw edge. Then measure and cut the piece I need.
If you don't want to piece your border, you can fold and cut the long way. If you end up cutting more than 4 layers, just make sure you have a sharp blade in your cutter, and cut slowly.