I have friends with a small Horse ranch out in the Phoenix, AZ area. Not far from her place is a large abandoned horse barn that houses a pair of Great Horned Owls (I'm not an Ornothologist, but that's what she says they are, and what they look like when roosting in an old tree in the distance).
Anyways, these two are HUGE!

She started having trouble with Roosters, chickens, and even neighbors pet rabbits and cats disappearing.

One night she was out with one of her dogs, and saw one of the Owls swoop into the fenced-in pen that the chickens and roosters are in. It is all Chain-Link fence, with a roof, but there is a gap at the top over the gate, and the owl got in through that, made short work of the rooster, and took off with it in it's talons. All that was left of her prize rooster was a bunch of feathers. I guess it really wasn't a fair fight, since it was over in about 15 seconds.
As for size, my Audubon book says that a Great Horned Owl is 25" tall, with a wingspan of up to 55". Actually, the Great Gray Owl is up to 33" inches tall, with a wingspan of 60". Wow!
