I don't know if there is such a thing...
Females are in general much smaller than males, for example I was at the humane society here today and they had five rats. Three hairless males (3m) and two females (American Blue berk and Mink berk, 1y). The males were bigger than the females.
And if you try and take into account dwarf rats... :lol:
I'd say that around a year they're adult, but I know some rats put on more weight when they're older as a 'cushion' against age, etc.