What this mix gives you is the shock-absorption of 78A's and the durability of 82A's. Skating style Speedskaters and aggressive skaters are the ones who use mixed durometers the most. Speedskaters do it for the hybrid durability and shock-absorption mentioned previously. Aggressive skaters often use the anti-rocker setup where the middle two wheels are very short and hard, and the front and back wheels are tall, to make rail slides easier. With the wheel diameter there isn't a whole lot to consider.
Taller wheels means more speed and the wheels last longer than shorter wheels of the same durometer.
Hockey players may opt for 70-72mm wheels, to keep their center of gravity down, but still let them maintain reasonable speed during play. Speedskaters usually go for the tallest wheels in the 80 to 82mm range. However this varies depending on the race situation. For example in sprint races, 76mm may be more appropriate. Comparing Core Types Cores can vary significantly in design.