Map size, while presenting a set of initial challenges, is not a large problem in this day and age.
Streaming, instancing, etc etc solves the performance issue completely.
The best clouds I have seen implemented in a current game are most likely the ones present in ArmA 3.
I believe they use a middleware which I cannot recall the name of now-- alas all cloud solutions based on real-world light scattering shader tech look quite alike.
