As for any variable, its type is found in its declaration:
EnumSet largeSize
So yes, largeSize (which should be named largeSizes since it’s a collection) is of type EnumSet. It should also be generified, and thus be declared as
EnumSet<Size> largeSizes
What it means, is that largeSizes is of type EnumSet. An EnumSet is a Set which contains enum instance of a specific enum type, in a more efficient way than other Set implementations (like HashSet, TreeSet, etc.). To know what an EnumSet is, read its API.