You will either have to put your birds up in a coop for a while or build a chicken tractor for them. Hawks are protected by law.
When November comes along, I usually coop my birds. I also have a big chicken tractor built from cattle panels and welded wire. Nothing can get in unless it digs in, and I haven't seen too many hawks that dig.
What you probably saw was a big female hawk. In hawks, the females are larger than the male.