How much do you want it to be? Certainly more than a dash, and well more than a pinch - neither of which seems the best way to measure butter, in any event.
A knob of butter is a British term denoting some butter, and its use is sadly declining as zealous editors force more precision and science into our recipes and cookbooks. Even the loosest British cooks (and we mean that in the nice way) might get away with telling you to add a knob of butter on a television program. But if their cookbooks are published in the States, you can bet someone will have translated all those knobs into precise measurements.
In our experience, a knob of butter is a couple tablespoons, more or less.