First, I don’t venture out so my breast size is a bit crazy. If I were to go out I would go for a 38B or 38C.
That said my go to bra size is 38DDD, but I also have 38G, 38H and 38M bras. I use water balloon forms for each size. I am 5’10” and 175 pounds so the 38DDD is large on me. the 38M is Bimbo level!
I also have a Roanyer breastplate they advertise as a “G” cup which is fairly accurate size wise on me. The 38DDD bra can not keep the Roanyer G cup under control, but the 38G bra I have is a tiny bit large. It may be the brand of bra too as different brands can fit differently.