i don't think it really goes by size, but more so by age and the quality of the environment/food, etc. my snails bred like rabbits if i kept them well fed and kept the water clean. a snail that is not fed or cared for properly all its life will never reach full size potential.
now i have about a dozen juvenile snails in a 55 community tank and some of them are about the same size as their mom when she laid her first clutch but none of the babies have started laying clutches yet, although i have seen the behaviours that they are breeding. these snails are approx 4 to 6 months old.
msjinkz! where are you? we need you on this thread! lol.