Yeah, I had to fix my post. The leghorn should increase egg laying not size. I have heard that Cornish birds have the best feed conversion, so they might be a choice later for better growth, still looking into it. How big do your BCM's get? They too could be a choice for size. I just want a good all around dual purpose flock for myself. I am not breeding for looks or to create a new breed of chicken. I do like how the BO and BA chickens look fat at full growth, that combined with their broodiness means I'll probably keep adding those to my flock. My SLW Roos have nicely developed chests, if they weren't so mean to my pullets I'd probably keep 1 for breeding. But disposition is important, I have to be able to handle the birds without fear of getting attacked by a psychotic Roo, and don't want hens injured either. Guess I will play around with breeds in my flock and see what I get. It could be fun, or it could drive me insane. Lol.
I think Barred Rocks (Plymouth Rocks) could be good for your plans. Old breed of chicken, good layers. I think I posted a vintage photo on that thread several months ago, let's see.....here it is. There are a few ducks in there too, but look at what appears to be a hen to the left of the photo...HUGE.
Size has been bred down by volume producers. You may be able to find someone with large birds on that thread and next Spring slip some eggs under a broody!