I know that by about three years of age a shubunkin goldfish will be 9-14 inches long and a comet goldfish will be nearly as long and a little bit wider...
thanks, now how many inches per month and what sizes do they start to slow down. I Just got 3 commons, one white and orange, one just plain orange, one orange with black fin tips. Also i got one comet that is orange with the tips of its tail white.
I don't really think there are any formulas for how many inches a month a goldfish will grow, it all depends on proper tank maintenance and feeding procedures. Keep them very clean and well fed and you will have huge goldies in no time.