I’d bought a body and a camisole in a local lingerie store (super-nice saleslady/owner was totally OK with measuring me and me trying things on) so I already new I was a 38. Then I saw a new bra and panty set in a charity shop in a 38D. Heaven. Next thing on the list was some D-cup falsies from eBay and I’m good go.
I went for a big cup size because a lady I know said, when I commented on her lovely boobs, they were a bit of a pain to carry round all day and I should try it. Now, after wearing mine for a day, I know some of how she feels.