have you measured the pattern according to your measurements? Sometimes the patterns do not measure the same. but keep in mind that when you measure you need room for give so it isn't skin tight. Usually this is 2-3 inches, sometimes more, depending on the pattern you are using.
If you know that you not going to use the other sizes at all, just lay it down and cut as you need to. You know where to cut, so you just pin it down and cut on the line you want. But................
If you want to save the other sizes, I usually clip and fold it under so I don't cut the other sizes off. But if you don't think you are going to use them, just cut where you need.
I hope this helps,
barb