The trout limit is 2 other than the Browns so any size brookie you could keep up to 2.
Having seen how nasty the brookie population could get in a stream if unchecked (unless you like catching lots of little 4 inchers), I'd highly recommend keeping as many brookies as you are legally allowed. The bonus too is how good they taste!
Having gotten my stewardship licence now, I'm heading tomorrow to the Ghost area to murder all the brookies I can find