i think the size of the terrier needed depends on the size of the earths you wish to work.
most of the earths i do are dis'used rabbit holes and my terrier often takes a while to get upto his quarry but i think as long as the terrier as the heart he will get there in the end most of the time
with a smaller terrier life would be a lot easier for me but thats life,i make do with what i've got and i have'nt be let down yet,so im happy