Inline skating wheels usually have 2 description: (mm) which is the diameter of the wheels and (a) which is the hardness of the wheel.
Big vs Small Wheels
Big wheels give you higher speed and better comfort when skating. However, it also requires more balance because it keeps you higher off the ground. Also, to hold larger wheels, the frame becomes longer, reducing maneuverability.
Small wheels give you better balance and it make turning easier. It is helpful when learning new tricks. However it significantly reduces the speed and users have to skate harder to cover distances.
Hard vs Soft
Hard wheels are extremely durable and it helps with speed. It is also good for tricks and slides. However, harder wheels also cause more vibration when you skate and it reduces traction.
Soft wheels are very comfortable to skate on and gives you better traction. However, the rubber on the wheels wears off quickly and the rubber gets compressed, slowing down the skates.