I think one cup full of rice is something like equal to that of a letter here in America. Though many countries use a letter system.
A- one cup of rice - 7 inches
B- two cups of rice - 8 inches
C- three cups of rice - 9 inches
D- four cups of rice - 10 inches
DD- five cups of rice - 11 inches
E/DDD- six cups of rice - 12 inches I presume that this is the size of Tsunade's
As for the number previous to the letters (which you'll find in bras) that number is considered ribcage/back&sides size. The letter itself pertains to the bra cup size.
It's funny how Kishi compares cups of rice to bra cup sizes. Technically, they are two kinds of cups yet they speak volumes!
Just google " bra size converter" and it will help to understand international sizes. And no, there is no rice size there!