KBS, all singletailed ("feeder", "pond", common or comet) goldfish can easily reach 18 inches in length. 9 inches is a healthy length for a 2-year-old fish, but a 3-year-old can be anywhere from 10-14 inches depending on how much you feed and the tank size. Based on my experience, I recommend singletail goldfish keepers start with a 75 gallon tank and then add 10-15 gallons per additional singletail. So your 90 gallon would be fine for two singletails. Beyond that, start looking for a larger tank. 
As a rough guide, healthy and well-fed goldies with plenty of room reach 6 inches in their first year, 10-14 in their second, 12-16 in their third, 14-18 in year four, and beyond that growth slows considerably to less than two inches per year. Koi grow much faster, since their adult size is much larger, and can reach 12 inches in their first year. By year two they can be anywhere from 15 to 25 inches.
As a rough guide, healthy and well-fed goldies with plenty of room reach 6 inches in their first year, 10-14 in their second, 12-16 in their third, 14-18 in year four, and beyond that growth slows considerably to less than two inches per year. Koi grow much faster, since their adult size is much larger, and can reach 12 inches in their first year. By year two they can be anywhere from 15 to 25 inches.
