Even within standard size breeds the size can vary quite a bit. My flock currently has 20 hens and 3 roosters. The girls include 3 cream legbars, an elderly sex-link, and the rest are F1 and F2 olive eggers.
The roosters are a cuckoo maran, copper maran, and olive egger. The boys are all pretty close in size, the girls vary a lot. Of the three legbars one is probably got at least 2 pounds on the other two. Three of my most petite hens come from the olive eggers.
I'd put the smallest hens at around 3 pounds, and the roosters probably average 7. I haven't had any problems with them injuring the girls.
The time to watch out for is when the hormones start really kicking in on your boy: even the ones that mature into good roosters tend to act like little terrorists when the hormones first kick in.
In my flock, they got the sexing right and I was able to integrate the cockerels into a flock of mature hens and guinea fowl, which meant the adults beat good manners into the little miscreants.