There are several factors that determine how fixable a dent can be for a Paintless Dent Repair (PDR) technician. Location, depth/severity, and panel material will play a larger role than the size will. There is really no limit to how large of a dent can be fix though since repairs are partially priced on size it can push a repair to not be cost effective.
Location of the damage has a major impact on how fixable and how difficult a repair will be. Sometimes a very small dent in a particularly difficult location is not fixable while a much larger dent in a better location can be fixed 100%. Some locations that cause limitations include body lines, roof rails and behind structural bracing.
Depth of a dent is a major factor in fixability. A BB gun or rock shot out of a lawn mower, for example, and cause a small but very sharp dent that can be difficult to repair.
Panel material is another major factor in determining fixability. Though most car body panels are made of regular steel, there is a growing number of panels made of aluminum, high strength and ultra high strength steel. These exotic materials are much more difficult to repair and will have a huge impact of the fixability of any dent.