I have a stupid question. Is a hen the full size and weight when it starts to lay or does she grow more? And the same thing with a rooster?
That's not a stupid question, but I'll have fun with my anwer.
The pullets will keep growing after they start to lay. They will gain some height and overall size, but a whole lot of the additional growth is spreading out. But no they have not reached full size when they start to lay.
On the other hand, I've never had a rooster start to lay before he reaches full size. But they will start trying to mate during their adolescence and well before they have reached ultimate size. Their saddle, hackle, and tail feathers will not be nearly as magnificent until they are a lot older either.
I have not studied it or paid that much attention to when they reach full size and glory, but at least for roosters, it is over a year and I'd guess more likely at least two years. It would not surprise me at all if someone comes on and says it is even longer.