It depends on your back size, if ur 34 then u just aim for 34d as 34 is the standard size.
If ur a 32, then aim for a 32dd,
If ur a 30, then aim for a 30e
If ur a 28, then aim for a 28f
As ur back decreases, ur cup increases by a cup whereas if ur back size increases then ur cup decrease by a cup.
X