The thing with cockerels, is they are a crap shoot. What you really need is a couple of plans. Because sometimes bachelor pens work well, sometimes they work for a while, and for some birds they won't work, no matter what you do.
As Ridgerunner states start with 4 ft sq for the coop and 10 ft sq for the run. Of course, more is better. Adding roosts, hideout, wind protection and multiple feed bowls situated so that birds can't see other birds eating somewhere else can be helpful.
Right now they are approaching the darling stage. They are brave and curious, and readily approach people. It is hard to imagine them not being darlings. But sometimes they change, and once they change, they do not go back.
I have kept a bachelor pen. Mine is not a forever pen. I would suggest putting the cockerels in there as soon as you know they are a cockerel, and do not want this bird for breeding. The bird you want for breeding should be raised in the flock.
Sometimes bachelor pads are tooted as a perfect solution for people that are very sensitive and cannot cull an animal. Sometimes they work for that, sometimes they don't. They don't call it cockfighting for nothing. Do have a 5 ft fish net, gloves, and at least a dog crate for plan B, set up and ready to go so that you can separate a fighting pair. Once they are truly fighting, I would not expect them to go back together well.
Mrs K