Hey,
You don't want a bra size for measuring bustiers and corsets. You need to look at the bust and waist measurements to ascertain if it fits :)
I'll give you and example using Sweet Streak Overbust Satin and Lace Corset-Style Bustier and my measurements. :)
I'm 38" around the fullest part of my bust, 26.5-27" around the waist and 35" around the fullest part of my hip. The exact measurements for these bustiers are detailed within the description, as they are with all Sweet Streak bustiers:
Small: Bust 31 inches / Waist 24 inches / Hip 32 inches
Medium: Bust 33 inches / Waist 25 inches / Hip 33 inches
Large: Bust 35 inches / Waist 27 inches / Hip 35 inches
X Large: Bust 36 inches / Waist 29 inches / Hip 36 inches
XX Large: Bust 37 inches / Waist 33 inches / Hip 41 inches
It also has a 5.25 inch modesty panel that allows the measurements to be expanded by up to 5 inches by leaving a gap at the back lacing. The modesty panel prevents any flesh being seen through the lacing.
The size large would give me a little bit of a squashed bust but would be inch-perfect on my waist and hips. This means I'd have to wear it closed at the waist and hip but leave it partly open at the back of the bust, which would look funny.
It would be better for me to buy the medium and wear it with a 2-inch gap at the waist and hip and then leave a larger 4-5 inch gap at the bust to allow me some extra room. My boobs will still be pushed 'right up in there' and I'll probably end up with one Hell of a chest shelf, but it would look better than one done up all of the way and just left open a bit at the top.
When it comes to cups, *much* of the lingerie we sell is MEGA mass produced and comes with a soft-wired cup that will accommodate a B/C cup, C/D in plus sizes. This is standard for Dreamgirl / Seven Til Midnight.
In the European brands such as Anaiis, the standard is for an A/B cup at size small with up to a C cup for size Large. This is due to the average sizing for women in the country of design.
It's not ideal but it fits the majority of women and that's why it's popular for mass manufacturers to stick with these standard wires. It's unusual to have a large proportional difference between the bust, waist and hip, after all! It's not much help to you or I, but it is the case.
At the end of the day, most of the lingerie we sell is designed to be worn for a sexy night with a partner, not everyday. When dressing up for a partner, many of us will squeeze things in because we're not going to wear it for long and it's about the look and the mood it sets. Lord knows that I wouldn't consider wearing most of my bedroom wear for more than an hour, I'd probably lose my breasts to bloodflow restriction. But it does the job of looking sexy, enhancing my figure... even if it is just for a few minutes. :)
Hope that helps! :)