A difference of just a week or two will make it hard to judge sex by size. Last year I had 4 whites in a pen..... 2 were hatched together 2 weeks earlier than the other 2 who also hatched together. At around 8 months I noticed that 1 of the younger ones was bigger than 1 of the older ones. At that point I knew the younger one was a male and the older one a hen. I ended up with 2 pair and I knew it at about 8 months.
Peep was almost twice the size of the other one which hatched at the same time and were brother and sister, and we thought were white. Boy were wrong Peep turned out to be a girl and both were BS. Oh and his sibling turned out to be a male. Here are some pix
Peep
Babies, Peep is on the left