First, measure a sweater that fits the way you'd like it to fit to see if it does have negative ease. Then knit the size whose finished measurement is closest to the one you measured. You know if your yarn stretches by knitting a sample of it, taking measurements (sts and rows per 4") before and after. Usually the gauge given in a pattern is after blocking, so that's the way to see if yours matches or not. When it does, then check it as you knit, matching the gauge you got before blocking so when you're done, it should turn out right.
FO means finished object.