heres one auto i grew at day 18 and then again at day 28
they will get bigger during flower. I Think these were around 7 inches tall when they showed sex. They ended up being aound 16 inches tall at harvest. Thats a very small autoflower though. I have had other ones that were probably around 12 inches at sex and then around 28 inches at harvest time. Dont stress it right now they are still young at 3 weeks old. They still have plenty of time to grow they will get taller and bush out alot within the next couple of weeks.