Hey, Steve -
In my use the medium needle works well for most everything, but if I'm doing something like a full primer coat or solid base coat, I'll step up to the large size. I only use the fine for very small area work, like wheels or camo borders, the fine is also good for getting into restricted spaces like flap wells, gear bays, etc.
While you could use the fine for overall work, the results would be second rate. When doing a broad surface area, it's important to have a greater amount of paint and wider area passes, so that you don't have paint drying before the next pass goes on. It's required to have each pass "blend in" with the previous pass, otherwise you will experience a gritty area where the edges meet.
I would imagine the fine tip would be marginally OK with say 1:72 fighters, but with large bombers, probably not. Much would also be determined by the thinning ratio of the paint, and the ambient temp when you're spraying
So, practice a good bit, experiment with thinning ratios, change needle sizes, soon you'll know what each needle size is best suited for. I'm inclined to think you'll find the med size best for overall use, fine for really tight details, like camo borders and very small surface areas.
Glad you like the purchase, let us know how you get on with the use of it.
Patrick