|Sandra's website no longer provides detailed information on how to select your size. This is some information I have gleaned, based on her handouts at seminars I have attended and experience.
Generally, she recommends to pick the size in tops that matches your full bust size. This is not always true. I will explain further below.
The cup size changes in each size range. ABC is B cup, DEF is C cup and GHIJ is D cup. So, rule of thumb: if your bust cup is different than the pattern, choose the size that corresponds to your high bust instead of full bust. You will probably have to make a bust adjustment but you will get a better fit in the shoulders. You may also need to adjust the side seams to get the width needed. Neither alteration is hard to do, compared with shoulder and neck alterations (in my opinion).
If you are buying the pattern for the skirt or pants, purchase by the hip size, comparing the pattern with your hips at their largest point. I find that for me, the waist on the pattern is usually very large compared with the hips, so I size down the pattern at the waist and blend the pattern line from waist to hip. You can also adjust the waist further by increasing the width of seams, darts, or pleats.
The pattern sizing is also designed for different heights, according to the size chart I have. ABC is designed for 5'3"-5'5"; DEF is basically 5'6"; GHIJ is 5'6-1/2". I mention this because this is a little shorter than the standard for the major pattern companies. If you are short and it's a toss up between size ranges, this might be another reason to choose the smaller size range. Vertically, things may line up better on your body.
Like me you might find that your top and bottom are 2 different size ranges, and of course, they are not in the same envelope (grrrr!). Joann and Hancock sales come in handy for this ($3.99 or 75% off) and I will buy both sizes. If you don't want to do this, I would recommend you buy the size that works best for the more complex garment, then adjust the more simple one. Complex and simple are relative terms and depend on your own shape! For me this would be the top. Also, the top usually has the more unique design, and if necessary, the pattern for the bottom can be adapted from another pattern that fits you.
I hope this is helpful!