This is really cool. If I remember right international passenger flight regulations insist that there be atleast one airport that can handle your class of aircraft within 180 minutes of flying time at one engine operative speeds at all times. If we assume a passenger jet flies at roughly 0.8 Mach or ~960 kph, and one-engine operative speed is roughly 40% of that or ~384 KPH. That means an airport needs to be within a circle of radius 1152 kms. That's an area of ~4.16M km-squared. Simplifying that all airports can handle all passenger jets, not one of those voronoi cells can have an area greater than 4.16M km-squared.

Is there an easy way to calculate the area of each cell?

Did I mention this is really cool!!