It is called easing....we all do it, a bit of a stretch here and there. Pinning at important intersections, using a walking foot and easing too short pieces work OK. If one piece is too long put it on the bottom next to the feed dogs...remember to always sew with baggy bottoms. Some times I will sew down a long seam halfway and then up from the bottom for the other half. However the thing that works the most is using a walking foot, especially on long seams like borders.