A “puck” of frozen food
The term "puck" is not in general use for food, but it seems to refer to a hockey puck-like volume of frozen spinach. (Spinach is commonly found frozen into blocks.) Presumably the author of the recipe had a particular brand - and therefore size standard - of spinach in mind. The spinach "pucks" in my freezer are about 4 oz each, but other sizes are also common. 

Find a better recipe, or ask the author for clarification.