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Good question. I wonder this myself too.
Yes there is a correlation on the average, a guy had calculated that from TP Datas.
From what I can remember, girth gains were about 0,47xlenght gains.
Starting stats (1st august 2011) > 16,3cm x 12,2cm
Short-term goal (1 - 1,5 year) > 18cm x 13,5cm