Hi,
Cockerels should weigh 2-3lbs at 8-12 weeks. Table birds should weigh between 3-4lbs at 12-14 weeks,but you can feed them up for longer,keeping them semi-confined ie not running all over the place will help,and you can go til 6-9 months. After that,I have found,they get a bit stringy.
Someone with more experience will come along soon and post killing out weights etc etc........ Ree