Misto is below. He too is not going to get much bigger. A very different dog in terms of overall shape, but about identical where it counts, which is chest size.
In The Working Jack Russell Terrier, Eddie Chapman writes:
"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. The biggest I can remember was a South Hereford fox that was one and a half inches smaller than my hand span, and that without my squeezing him. It therefore follows that if I can pick up a dog and just span him with a squeeze, then the dog cannot get to the fox in a tight place and a dog that cannot get to a fox cannot be considered a Jack Russell. Either you are breeding a terrier suitable to work fox or, if he is too big to get to a fox, you are just breeding for looks. This is, of course, what happened to the pedigree Fox Terrier and look where that has got him!"