I don't think space is the problem. The reason we all mention it, is it is the most common problem.
I would pull the aggressive bird, see how the flock acts, generally one of two things happen. Your flock is peaceful in a few days. Then you can add the other bird back, see if it is a go. If not, then she should either go, or try the pin less peepers, I have heard of good results with them by other posters.
The other thing might happen, is that anew bird will step up to be the bully to the victim. Then you might need to get rid of the victim. There is a theory that chickens can sense an illness or a health issue well before humans do. I myself had this happen, and with in a few days of this behavior, the hen was dead.
So frustrating,
Mrs K