For Those Adept and European Cookery

Yes, I'm looking at you, desperance, but anyone else, please feel free to jump in.

I found a lovely recipe for tomato-less spinach lasagna, with four cheeses (ricotta, parmesan, mozzerella, and gorgonzola), bechamel, and spinach (couldn't find the nutmeg, so used goram masala, which seemed to work).  I have no problem getting measurements in metric.  Had the nice person who posted this recipe on the internet remembered to note what temperature to bake it at, I would have had no problem translating that (either to centigrade, or to the 1, 2, 3 etc. settings).  It is in the oven now, and good smells are issuing forth. After the pans are all cleaned or in the dishwasher for later attention by my mechanical minions, I am left with one question:

How big is a "knob" of butter?

In the event I threw a chunk a little less than a tablespoon or so into the pan that I was wilting the spinach in.  But I'm curious.