Dr. Paolo Cavallini measured over 300 red fox it Italy and compared them in size (weight, length, and chest size ) to red fox in other countries of Europe, Australia, and the U.S. His paper was published in the December 1995 edition of Ann. Zoological Fennici in Helsinki Finland.
Italian foxes varied from a minimum weight of 7.2 pounds and a minimum chest size of 10.43 inches to a maximum weight of 17.9 pounds and a maximum chest size of 18.3 inches.
On average, Italian foxes weighed 12 pounds (plus or minus 2 pounds) and had chests that averaged 13.8 inches in girth (plus or minus an inch).
Dog foxes averaged 13.8 pounds in weight (6,283 grams) and vixens averaged 11.6 pounds (5,265 grams) with average chests of 14.13 inches for dog fox and 13.27 inches for vixens.
Chest size did not vary much by region in Italian foxes -- most of the variance was in body length, and weights seem to move up and down on that basis.
Italian dog foxes average two pounds lighter than their English kin, and Italian vixens averaged one pound lighter than their English counterparts. Though you would think this might make for an English fox with a slightly larger chest, this does not appear to be the case -- again body length seems to the determinant variable.
Barry Jones, terrierman to the Cotswold Foxhounds, a former Chairman and President of the Fell and Moorland Working Terrier Club, and the founding Chairman of the National Working Terrier Federation writes:
I have not encountered a fox which could not be spanned at 14 inches circumference - this within a weight range of 10 lbs to 24 lbs, on average 300 foxes spanned a year.
Eddie Chapman concurs. He writes in his excellent book The Working Jack Russell Terrier:
I am a small man and have reasonably small hands, but in more than 20 years in which I have handled well over 1000 foxes, I have never handled a full grown fox which came anywhere near the span of my hands.
Bottom line: If your dog has a chest larger than a fox, it will not be able to reach a fox in a tight earth. Anyone who talks about "heart" and "desire" being a factor has not dug enough animals yet. Heart and desire carry no freight with rock, root, or 15 feet of baked clay.
Folks who have dug three or four or ten animals and now think they "know" terrier work are simply waiving the red flag of inexperience. As C.S. Lewis once observed,
Experience is the most brutal of teachers. But you learn, by God you learn.
Larger dogs are a dime a dozen, but they can only be used in a fraction of the holes.
Look for the smaller dogs, and you will never need a larger.