DoubleCK, I dont know much about the breeding of Den Terriers. I know they all look a lot like JRT's, and im sure that they are some cross of JRT.
The Blood line of JRT's that we bread and kept were known to produce some real small dogs, around 10-12 lbs.
I Never knew much about hunting with terriers until a few years ago, i just used mine when the opportunity came along, so i had no clue that those little ones could be so valuble.
There is a guy up in Michigan that is raising some around the 6lb mark from what i hear.
From what i understand, and maybe someone else can take this farther, or corect me if im wrong, But the idea of a Den Terrier is to ether push the game out of a hole, or hold it until you can dig to it, and get some bigger dogs on it. I dont think they actually engage the game, but may be wrong, im sure i wouldnt want a 6lb terrier fighting with a big red fox in a hole that i couldnt get to.