here's some information that might help you! ! ! !
For the USA:
Our longest paved runway is Denver International's 16,000 footer

Our (and the world's) longest runway is Groom Lake (Area 51!)... a real whopper at 27,000ft!

As for the largest commercial runway in the world:

Ulyanovsk-Vostochny International Airport (Russia) at 16,404ft

Heck, just make your own! Search the sim sites for an older program called Runway 3, it is the absolute simplest program I've found for creating and placing a runway anywhere in FS. I run FS 2002 and am too lazy to use AFCAD or dive in too heavily to scenery design programs but this baby lets you choose grass, tarmac, dirt or concrete, Length and width.  You simply hit shift + z while parked where you want the runway to be and record the coordinates text that appears on the screen. Then pop out of the sim, open up runway 3, type in the coordinates and your runway preferences, hit "create runway" and voila, there she be when you fly again! The program tells you that your new runway is saved as 'RUNWAY.BGL' and to rename it if you don't want to overwrite it when you create your next runway. I also recommend another freeware download called 'world airport upgrade', it just makes all the runways and taxi lines, etc look more realistic globally (Tire marks, better textures, markings) without any noticeable framerate hit. And it seems to render your own runways the same way. Runway 3 will ONLY create a runway, though, you need to find an appropriate object placer program to throw in buildings, vehicles, etc.  PS. if you screw up and try to create a runway going into water or something funny, the program just says "overflow" and won't create it, try again. I've had a riot putting airports onto some 10,000' mountains in western Canada. Worth the download, just make sure you install it according to the readme file. (Easy)