Paniculata Bobo is quite compact and very winter hardy for northern gardens. It also blooms on new wood so there is no winter damage to be typically afraid of. But if you are looking for mopheads, check the Let's Dance and Cityline Series of compact hydrangeas. They may not be as reliable bloomers as paniculatas but they will perform ok in Zone 6 and warmer ones. Winter protection applied in the Fall will usually make sure that you get bloomage; needed usually in Zone 5 too.