Here is the link to an earlier discussion on here about cowboy installers with a few useful hints about chimneys etc
Your duct should extend downward from the ceiling to close to the bottom of your woodburner so that you are keeping the hot air inside the room. Your total vent size should also take into account any VMC or other extractor fan(s) you use in the kitchen and W.C/bathroom.
It might be a leaky old house ATM but almost everyone wants to draft proof in the winter!