The present work explores the effectiveness of depositing drug particles into the human respiratory airways to cure pulmonary-bound diseases by employing the Eulerian thin film and discrete phase methods. In the context of respiratory disorders, pressurized metered dose inhalers, nebulizers, and dry powder inhalers are widely utilized for medication. This work assumes inhalation of the tiny particles of liquid drug compositions, which create a thin layer of drug spread when colliding with the airways’ wall. Parametric variations of the investigation consider two different particle sizes of 10 and 5 µm to compare and investigate the deposition efficacy. The study reveals that the size of drug particles significantly influences drug delivery effectiveness. As a result, the total fractional area covered by the 10-µm-sized particles is around 28.94% higher compared to the same of 5 µm. Due to the stronger inertia force, larger medication particles have better fractional area coverage in the initial zones of the airways. However, the smaller particles ensure better drug penetration into the deeper generation regions of the lungs, which is around 17.4% higher volume fraction for the 5 µm particle sizes. Therefore, an appropriate mixing of particle sizes is essential for the overall success of particle-based drug delivery techniques.