There is no upper limit to 3D min or max distance since the values are in your own units (except of course for the limitations of a float).
Yes you can change these values at runtime, and keep in mind that min distance is when the sound starts attenuating and max distance is where it stops attenuating.
You can set the min and max at runtime with FMOD Ex via Channel or Sound ::set3DMinMaxDistance and you can set it via the Event System with Event::setPropertyByIndex and use FMOD_EVENTPROPERTY_3D_MINDISTANCE or FMOD_EVENTPROPERTY_3D_MAXDISTANCE.