I think it would be too complicated to bring resources into the equation.
As for minimum size of nation, yeah, we'll have to figure something out for that. Then again, it's possible for a tiny nation to attack a large nation. I think what would be a better idea is to say that the size of your population (in members) or else size in territory would influence not your chances of winning but how much territory you would gain. That way, while a small country can take over a neighbouring country's town, it would be seen as possible suicide as it would have angered them into wanting to attack back.
Of course, having said that, the chances of a large nation having no members active in 24 hours is pretty low, and the chances of a small nation having no members active in 24 hours is much higher. Again, it would be suicide. Likewise it would be daft for a large country to attack another large country, because they'd react just as fast.
And, again, this would all encourage activity and development of the world and nations rather than encouraging war, so that's cool. Activity = Survival, and that makes sense, really.
