Add a Knob of Butter

I was trying to fix some chestnut and cranberry stuffing this weekend and ran into a measuring problem. The instructions on the box of Paxo stuffing mix from Britain said to "add a large knob of butter". I'm not sure if a "knob" is a standard unit of measurement in the U.K. but I couldn't find a "knob" listed on the side of my measuring cup. Did they mean the size of a doorknob or more like a dime-sized knob? As the British would do I just muddled through and it turned out ok.