Sirkitbreaker, just before the topic closes, let me add my two bobs worth.
Topunicyclist has got the right sort of dimensions. For single engined aircraft under 2000 kg MTOW, runway width 10m, plus 10m strip either side (which is not-too rough ground that the aircraft can run on in an emergency), and 15m either side of fly-over area (rough but clear of trees), for a total cleared width of 60 metres. Length of 1000m is typically fine; some people use a bit less especially with STOL mods. 1200m is often used for twins. That Australian CAAP 92-1(1) is excellent, and is built on decades of bush experience. Let me know if you want a copy.
The three practical things to watch out are:
Rough surface
Soft, wet surface
Takeoff gradient.
Roughness is tested by driving over it in a laden 4wd or utility at 75/80 km/hr (as Tinstaafl suggested). What I do is use the personal car of the local manager/owner - it focuses their mind wonderfully on the strip condition when they have to drive it in their own precious vehicle.
Soft, wet surface is tested by driving over it in a zig-zag pattern at very slow speed and looking for wheelruts more than 25mm which indicate soft areas. Maybe give a couple of spots a dig with a crowbar to check that it is not just a hard dry crust over a soft base (absolutely essential if you're in an old lake bed or there are white salty patches around). If in doubt, borrow a 5 tonne truck and drive the runway using that. If it doesn't get bogged, your 2 tonne aircraft shouldn't either.
The takeoff gradient can occasionally be tricky if the ground is generally sloping. You're looking for maximum 5% clear gradient (which is about 3 degrees). I've been caught just the once where a strip approach looked reasonably clear to the naked eye, but the whole landscape was sloping and the actual takeoff gradient was 11%. If the aircraft had ever got off the ground, it would have hit the rising ground further along. The only way to be sure is to use a small clinometer (costs about US$60), but you can make your own. Got this cute method from a primary school, using a straw, string and protractor.
http://www.zip.com.au/~elanora/tclinom.html
Oh, and finally, when the guys who are building the airstrip reckon it's completed to standard, there is usually a few more days work needed to finish it.
