Mollies usually become fully grown at 4 months. Do you happen to know what species your molly fry are? Also if they are in cramped quarters it can stunt their growth, did you have them in a breeding net?Also if your mollies are inbred it can stunt their growth.Stress from constantly being chased by adult fish can also stunt their growth.Colder water makes molly fry grow slower. Warmer water (around 80 degrees) makes them grow normally Shortfin mollies only grow to be about 3 inches for males, 4 inches for females. Sail fin mollies can grow up to 6 inches .Balloon belly mollies it can be hard to tell since they are so deformed and so hybridized but the general rule of thumb is 3-5 inches.