Hi Fee,
You don't need to cut all the same size, nor are you intended to try. You can blend between different sizes at the bust, waist, and hip. Also, and this is important to fit, the bust size depends on her bra cup size. Have a read through the "Sizing Your Pattern" instructions on page 2 again - you want to start with the right cup size line (A - DD) and find her bust measurement on that line only. That's the size you should use for the bust. Then pick the waist and hip sizes independently of the bust size or each other - it sounds like you should definitely cut size B for both. If the bust is a different size than the rest of the corset, you'll need to trace the corset pieces off, blending smoothly from the bust size you've chosen to the waist size you've chosen.
The reason cup size matters is that someone with a B cup and someone with a D cup could have the same bust measure - but the D cup person would have a much smaller ribcage and back measure, and much more breast mass. The B cup person would have a larger ribcage and less breast mass. The D cup corset would gape in the front and pinch in the back on the B cup wearer, and the B cup corset would give the D cup wearer quad-boob and would flap around in the back.
Make sense?
