When is a barrel length too small that it affects performance of the marker? and what is the optimal barrel length?
The optimum length for barrels is 14", one piece un-stepped barrels are best at about 12". Remember this is not accuracy, this is efficiency, too long a barrel and you increase drag and decrease efficiency, too short a barrel and the gas doesn't have enough time to propel the paintball properly so you decrease efficiency.
Depends on what your doing. But 14 inch is about the perfect all-rounder.