[quote]WrenWright
<Any ideas about how large a prop I could swing?>

I wonder if you're really asking the question you want answered? If you treat, and tune, the Dragonfly like a Texaco engine as suggested by ptulmer then you can swing up to an 8" prop on a practical basis. But then you must fly the airplane as a Texaco time endurance competition model would be flown, i.e.: slow and gentle climb to speck-out altitude in order to maximize time aloft. If that is truly what you want to do then both engine and model should lend themselves well to the effort. A carefully tuned Texaco engine with the large tank, running a wide-blade 8" prop will easily exceed 5 minutes run time so the larger tank on the Dragonfly could concievably provide near 10 minutes of mild-mannered run. But since even the standard Texaco tank will take a model higher than it can be seen I'm afraid I see no great benefit in a longer run.
But if you are under the impression that a bigger prop will generate more power for high speed or high thrust applications then you are setting yourself up for disappointment. Ultimately no matter the size of the prop you have only so much power to turn it. Or, in old hot-rodder parlance: "There ain't no replacement for displacement".
If Texaco type performance as described is what you seek then you should be able to find reams of information about how to tune the Cox engine on this site with a bit of diligent searhing.

<Do I HAVE to use a 100% castor fuel?>

Only if you want the engine to last.

Dunno if that helps, but there it is. Tom