If you are talking about the pieces being too short, I was taught to measure the longest and widest part of the piece I am about to add and add one inch to the fabric.
Also I prefold all my paper before starting to paper piece. It takes some time but is so worth it. That way you can actually fold the paper back to see if your fabric will extend far enough past the ends of the piece you are adding and allow you to adjust before stitching.