The control zone for individual airports as shown on aeronautical charts has nothing to do with the radius of intended coverage in my opinion. In the case of individual control tower frequencies, these are usually accessed by pilots flying VFR when well outside the airport's control zone. Unlike larger airports which handle almost exclusively approach/departure controlled traffic, the small airports handle mostly VFR traffic.
From my experience it appears that 20 to 25 statute miles would be a better radius of intended coverage than going by (mostly smaller) control zone maps - at least for the tower, ground and FBO frequencies. Ground is really short range but falls under the Air Traffic Control 'department' so would necessarily share the range of the tower's frequencies. Often the FBO takes orders for wheelchairs or other special handling and fuel requests while the incoming aircraft is still 15 - 30 minutes out. That translates into quite a distance even at 200 Knots (230 MPH). 15 minutes at 200 Knots is 57.5 miles.
I'm leaning toward FBO ranges of up to 50 miles... Most FBOs use upwards of 25-50 watt radios and most commercial planes have 25 watts I believe. Private planes are around 10-15 watts and hand-helds about 5 watts. The 118-137 MHz frequencies carry well line-of-sight and if the aircraft is at altitude; even further.
This sounds like a difficult problem to come up with a single "right" answer to.
