Tu ne crois pas si bien dire is a common phrase that means “You are definitely right, even more than you could imagine”. You have to understand this phrase as “you don’t believe that you are so right when you are saying this”.

As for figure, I understand it here as as figure de style, which has the same meaning as figure of speech in English. As Patachou tries to speak like a poet, he uses figures of speech (eg metaphors) without even knowing it.