I have built and mapped a couple of new bases so far that are wip, these being NAS Fallon and MCAS Yuma. I found that the easiest way is to initially compile a large screenshot of the base using an imagery service such as google earth, I usually make my 'plan' picture several thousand pixels tall and wide. Then in max, create a plane that has the same proportions of your large image. Next I would make the runways out of planes, creating them using the keyboard input with the appropriate values in max. Using the runways as a guide, you can then scale up/down your plan pic to the correct scale. Next, I move onto the ramp areas, for which I use the line tool and then extrude them flat. Once this is all done, I will start to map the areas, making sure that all materials are set to Planar XYZ. The only other major part of building the airbase after that is possible creation of new objects and then placement of the objects (hangars, revetments, vehicles, oil/water tanks etc.)
This is quite a very rough guide, if you need more details, please feel free to ask.