Some recipes, especially drink recipes, call for a "jigger" of an ingredient. Exactly how much is a jigger? In terms of measurement:
1 jigger = 1 1/2 fluid ounces
However, there are also pieces of bar ware known as jiggers or double jiggers. These usually have graduated marks denoting the measurements in ounces or if not marked for specific measurement hold between 1 and 2 ounces.
My rule of thumb when it comes to mixing cocktails: if you're unsure how much of the ingredient to add to the drink, start small and mix to your taste!