The British Connection
In the UK, the English Working Cocker Spaniel was introduced as the Stud dog. This was later varied using the English Show Cocker Spaniel and the American Show Cocker Spaniel, all of which have distinctively different coat types.
The Cocker Spaniel half of the equation also introduces a degree of variance because American Cocker Spaniels are slightly smaller than their English counterparts and have a longer coat. In the UK, generally the Cockapoo is 1 - 2 inches bigger than its parents.