I have asymetrical forms – they’re similar to triangle forms but have a bit of extension that goes toward the underarm. Mine are the perfect size for my 40 E/DD and 42 D bras. That’s pretty large but look proportional on my body.
One caution. You should buy bras to fit your breasts/breast forms. Not the other way around. It’s difficult at best to find forms that perfectly fit a particular bra you’ve already purchased. Your best bet is to talk to a rep at a reputable breast form store to find forms that are appropriate for your body. Then get fitted for a bra at a local lingerie store.