This study investigated how varying the dosing cup size of a novel reservoir dry powder inhaler (DPI) affects the detachment of a micronized active pharmaceutical ingredient from larger carrier particles, and the aerosol performance of a DPI carrier formulation. Three different-sized dosing cups were designed: 3D printed with cup volumes of 16.26 mm3, 55.99 mm3, and 133.04 mm3, and tested with five different carrier type formulations with beclomethasone dipropionate (BDP) concentrations between 1% and 30% (w/w). The morphology of the BDP attached to the carrier was investigated using scanning electron microscopy and the aerosol performance using the Next Generation Impactor. Increasing the volume of the dosing cup led to a reduction of BDP deposition in the Next Generation Impactor preseparator, and an increase in BDP detachment from the carrier was observed, leading to increased aerosol performance. The decreased amount of BDP attached to carrier after aerosolization was attributed to the increased dosing cup void volume. This may enable greater particle-particle and particle-wall collisions, with greater BDP detachment from the carrier and deagglomeration of smaller agglomerates. The dosing cup volume was observed to have significant influence on particle dispersion and the overall aerosol performance of a DPI.