Cocker Spaniels were first recognised by the Kennel Club as a separate variety in 1873, before this concession was made, it was customary for dog shows to put on classes for Field Spaniels, meaning those that worked and not a separate variety. They have been and remain amongst the most popular breed of dog world wide.
Traditionally Cockers were the small ones in a litter, the larger ones being put into competition with classes for heavier dogs, in some cases even one of a litter might be classed as a Sussex Spaniel the rest being classed as Cockers, such was the ambiguity of the breed in those times.
If you sat in your home laboratory and tried to create the perfect dog, you would obviously be better off taking a trip to the local Labrador Retriever breeder, where you would find a top dog waiting for you.