I have dug on fox, measured them, released them, and photographed them. These two pictures are from my yard last night. Here's the short story as far as size and food: this is a very small predator.
Fox mostly eat mice, but really will kill your free-range chickens and ducks if you leave them out at night or fail to maintain a fox-wire surrounded coop (not chicken wire, fox wire!).
Fox will generally ignore a cat, cannot kill a sheep, and outside of mice, they mostly live on scavenged berries, bird seed, roadkill, frogs, snakes, and the occasional bird (they are called a cat-like canid for a reason).
Fox have a chest size of about 14 inches or less, and larger weight fox tend to be longer, but not much bigger in the chest.
A dog that has a 14 inch chest will be about two inches shorter in stature than a fox, which is mostly leg, with a bone structure closer to that of a bird than that of a dog. Look for a small-chested 12 inch tall terrier (or less) and you will never regret it.
The far-and-away biggest killer of fox is disease (distemper, mange); parasites (roundworm, hookworm, heartworm); exposure, flood, and respiratory illness (as kits); starvation; and happenstance (ripped by barbed wire, caught in brambles, accidental poisoning from antifreeze or rat bait). Encounters with farm dogs prune of some fox, and vehicle impact kills many others. Trappers and hunters have almost no impact on fox numbers at the national level.