You will work out the measurements you need by first setting your trap, this will give you the width, do not forget to allow for three quarters of an inch for the safety catch, then release the trap and you will have your height, allow about three quaters of an inch above that, the tunnel itself should be around two and a half feet long if it is going to be a run through or about eighteen inches if a dead end.
Hope that is of some use, do not forget to push a couple of sticks in front to stop larger and non target species entering the tunnel.
What mole trapper said but don't make the