Sorry, but upon further calculations, my initial estimate for the number of holes is off by several orders of magnitude. There are apparently at least two different types of PSP used in WWII. (Don't jump on me fly-boys, I'm only a tread-head here.) The photo of an actual piece of PSP I found on-line says it is 19 1/2 inches wide by 11feet, 9 and 3/4 inches long. ( http://www.sml.lr.tudelft.nl/~home/rob/models/psp.htm) Rounding this off to 12 feet long or 144 inches, makes a piece in 1/72 scale come out to 2 inches. A 16 foot long model would be 192 inches, divided by 2 equals 96 sections of PSP for the length. The 1/72 scale PSP would be .27 inches wide, so dividing 15 inches for the width of the runway by .27 equals 55.55, so roughly 55 sections of PSP for the width. Each section of PSP has 160 holes in it. 96x55 equals 5280 sections, this times 160 holes per section equals 844,800 holes. Sorry, I didn't see any size listed for the holes. Once you get the correct size, you better start drilling!
We'll check back with you in a few years to see how you are doing!
If you really want to make this large of a runway diorama without spending most of your life drilling holes, there is an easier way. Buy either the photo-etched PSP or better yet, the resin PSP runway sections, (There are several companies that make PSP, Verlinden, etc.) and use it as a master to make a mold with either latex or RTV silicone rubber and then cast sections of runway from this. This would still be a huge task, but far easier than the alternative.