Decades of Style Pattern sizes are based on a long held industry standard, using a B-cup full bust measurement. This measurement gives a close approximation of your frame size. The hardest area to adjust is around the neck and shoulders. Using your full bust measurement will get you closest to the fit in this area.
For a blouse, jacket, or dress pattern, choose the size based on your full bust measurement, not your bra size. For a skirt or pant pattern, choose the size based on your waist and hip measurements. It is always easier to adjust a pattern larger than smaller. Check the finished pattern measurements. The finished pattern measurements will also tell you how much ease is built into the pattern.
If your bust is larger than a B-Cup, you will get a better fit in the back, neck, and shoulders by choosing the size of your pattern by your high bust measurement. This measurement is taken by passing the measuring tape across your back, under your arms, making sure to pass over the shoulder blades and resting in the front just above your full bust. Use this measurement as your bust measurement when choosing the size. It's important to fit the shoulder area first, because this area is harder to alter on a pattern than the bust, waist, and hip. Check the finished pattern measurements printed on the pattern to see if you need to increase the pattern in the bust area. Many styles have enough ease built into them that a bust adjustment is not necessary.
If your bust is smaller than a B-Cup, and you have a large frame, you might want to check the finished pattern measurements and choose a pattern that is one size larger than your full bust measurement.