I'm terrible at sizes but I have a small back garden around 40feet wide by 60feet long. My intention is to have 2 ducks and 4 chooks in there. Without a cockerel to keep them in check chickens can be quite cliquey so I'd suggest getting them all from the same batch so you don't have to worry about if they'll get on.
I had two dogs with my chooks and the chooks ruled the roost, had to let the dogs out into the garden before the chickens were out in the am or the dogs would refuse to go into the garden. One was a lurcher (you wouldn't think chickens would last around one but no issue, the other was a lab). Consider your dogs temperaments, how do you think they'll be around the chickens?
Raptors! Are there any around? We once had a buzzard try to take our bantams.
Garden secure? Local dogs broke in and killed most of our chickens. It was the reason we stopped, we couldn't stop them breaking in.