Arielle,
I weighed a couple of the White Cornish chicks today, one at just over 3 lbs and the other just over 4 lbs. I have not banded chicks as to hatch date, but lost the entire pen of chicks that were all over three weeks old and up on May 20, so these are my oldest I now have and hatched in April at the earliest, probably late April. [I had moved everything that hatched sometime in April, between 50 and 60 chicks, that were feathered well enough to leave the brooders, to an outside pen. What turned out to be a badger burrowed in and carried off nearly every chick, including all of the Cornish that were outside at that time.] I have some in that pen that are smaller, some a bit larger. and three groups still in brooders though some need moved soon..
If you are interested in raising quality Cornish as your choice of a heritage breed I do want to encourage you to get in and stay in. On the other hand it is much easier to get started with and keep a flock of many other heritage breeds. While I love heritage breeds and Cornish in particular, and consider Cornish the ultimate meat breed, the easier and often cheaper thing to get started with are either hatchery sourced breeds or commercial lines. Some say you have to be a little crazy to breed Cornish, and I admit if I just wanted to put meat on my table I would not be breeding them. I keep other chickens as a source of eggs, and could buy both meat and eggs cheaper at the grocery store if I wanted. There are many heritage breeds well suited to provide both meat and eggs, but I love large fowl Cornish.