Girls! I need your help understanding dress sizes?

My girlfriends birthday is coming up and there's this dress she's always wanted which I'm planning on getting for her birthday.

The difficulty I'm having is that I know she was a size 4 but a couple of months ago she had a breast augmentation which took her from a 32B to a 32DD and now I have no idea what size dress is suitable to accommodate her new larger breasts.

And I don't want to ask her as she will realise that I'm planning on buying her that dress she wants and spoil the surprise. Please help!
Girls! I need your help understanding dress sizes?
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