I got lucky I guess. I moved from a suburban sprawl in massachusetts to rural puerto rico two months ago and the two year old bichon frise I brought with me, herds my birds. never trained her, never even thought about it. At first i thought she was gonna run them down and kill them, but she just scares them into running in one direction by sorta bouncing from side to side in a zigzag form as she runs behind them. the only flaw are flighty cowards. instead of running with the other birds a couple of hens have desided to just get up into a tree instead of dealing with the dog.
I would think a smaller breed would be ideal for hurding chickens. They are faster, more energetic, no less threatening to the chickens, and even if they wanted to kill a chicken, it would be considerably more work than for say a German Shepard.