One of if not the biggest Corydoras species is C. robustus, the largest member of my group measured 105 mm SL. (4.25"). There is of course Scleromystax barbatus, which is also in the family, also grow to a similar size. To answer you question, "how many months it takes for a Cory to grow to full size" is a little like asking "how long is a piece of string" C. pygmaeus can reach adulthood and be fully grown in less than a year, but when it cones to the larger species you measure in years not months. I have C. sterbai that are at least 14 years old and they are still growing, albeit very slowly. The largest female is now 65 mm SL' (2.62"). SL = Standard length (Body length) If you count the caudal fin (tail) then that is the total length and then for all the above you can add at least another 13 - 14 mm (9/16").
Ian