Dinked

You can estimate the distance by going to the FMB and placing one plane with the player box checked and making two waypoints on opposite sides of the map.  Go and "test" the mission but on the mission introduction page, zoom into the first waypoint.  It'll give you the distance to the second waypoint and thus the approximate size of the map.
 
See attached image.

Sounds like the AI got tweaked from when I put that campaign together or maybe it's the particular plane in question.  The fix is to add the airfield to the airfields.dcg file and set the take off waypoints such a way that they can through any surrounding hills.  I can post the fix if you let me know which airfield it is.