The dimensions you need aren't stated specs. You'll have to either physically measure the current spokes and/or the rim ERD and the hub dimensions or email Dahon and see if they can provide you with specs.
Spoke durability is most prominently a feature of build quality--high, even tension and appropriate stress relieving during building, but generally butted spokes will be more durable because they are more flexible in their centers where they are least likely to develop fatigue failures and thus keep the elbows and threads of the spokes from flexing and developing fatigue. The most durable are probably triple butted spokes with a 2.3/1.8/2.0 profile like DT Alpine III or Sapim Force (2.18/1.8/2.0). At lower cost you can also just build with common double butted spokes (2.0/1.8/2.0) or single butted spokes (2.3/2.0).
If you've had a single spoke failure I wouldn't stress out too much about it. 200lbs on a 20", 36 spoke wheel sounds perfectly reasonable (smaller wheels are stronger). Factory wheel builds are typically not very even in tension and it'd be premature to assume many other spokes in the wheel have undergone significant fatigue. A good intermediate step would be to have a good wheelbuilder not just true the wheel, but bring it up to maximum balanced tension and stress relieve the wheel.