Jiraiya stated that Tsunade's oppai size was 106cm.
From Tsunade article:
Despite being known as flat-chested in her childhood, she now has a rather large bust; 106 centimetres in circumference, according to Jiraiya.
From Jiraiya article:
when caught spying on Tsunade in his younger years, she proceeded to break both of his arms, six of his ribs, and ruptured a number of organs as punishment.
Considering the amount of fear and respect Jiraiya then developed towards her, how did he know the measurement of the adult Tsunade?
(I'm really sorry about the quality of the video)
Below is the image from the corresponding scene in the manga (chapter 406, page 9):