The stretch makes these good garments to fit the varieties among our bodies. The sleeves and chest are true to fit, while the body of the sweater falls a bit more loosely. If you find that you fall in the middle of two sizes, I recommend that you err on the side of smaller as the sweater is meant to have a slight natural flow.
The best way to take your measurement is to use a measuring tape and find the size that best meets the averages of these measurements. When measuring bust, measure wearing a bra you would wear with this garment.
Another helpful frame of references is that I am an 8 in Opaline Hue and find that most of my clothes in my closet are a medium or size 8. I recommend ordering what you might generally gravitate towards.
If you have any questions about choosing a size, please contact firstname.lastname@example.org with your measurements.