There are two dimensions: band size and cup size.
Band size, in the British system (there are several) is the circumference above the breasts in inches - or circumference below the breasts + 4 inches.
That's relatively simple.
Cup sizes are a more complex topic. They are indexed by letters but a letter alone says nothing about the breast's size.
Cup sizes depend on
difference between the band size and the measurement at the fullest point of the breast. The volume of a cup of the same letter increases with band size - roughly when you decrease the band by one size, you need to increase the cup by one size to save the volume.
An example chart:
Oh, okay. Thank you.