Take -> chest circumference (i.e 35") and divide by 2 to get width = 17.5"
Remember sweatshirts are a little bigger than shirts so add 4-5 extra inches = 22inches (approx.) = size medium
If Jamie wants a snug fit she can get a small, if she likes a really baggy fit she can get large. But since Jamie likes a relaxed fit, and she normally wears size 6, she got size medium.
Length = highest point of the shoulder to the bottom hem of the shirt.
Width= across the bust 1-inch below the armhole, seam to seam.